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TN 



Stamp Milling of Gold Ores 



* a . . r 



V' 



A BY 

T. a: rickard, 

Mining Engineer and Metallurgist; 

Fellow of the Geological Society; 

Associate of the Royal School of Mines, London; 

State Geologist of Colorado. 



SECOND EDITION. 



NEW YORK AND LONDON: 

THE SCIENTIFIC PUBLISHING CO., 

1898. 






.» 






OOPYKIGHT, 1897, 
BY 

THE SCIENTIFIC PUBLISHING COMPANY. 



I 

V. 



THIS BOOK IS DEDICATED 
TO 

RIOHAKD PEARCE 

AS A TOKEN OF RESPECT FROM 
THE AUTHOR. 






This book offers a careful description of the milling practice 
of gold-mining districts on both sides of the equator. It 
records the results of investigations made by the author in 
the course of professional work during the past ten years, 
and is published with the hope that the data which it contains 
may prove of service to comrades in the same field of industrial 
endeavor. 

The author's inquiry into the principles of this important 
branch of metallurgy sprang from the first observation of the 
apparently contradictory practice of Colorado and California, 
in both of which states (during the years between 1885 and 
1888) he had charge of the operation of typical milling plants. 
The search for the scientific principles explaining methods so 
diverse led him to the general study of the reduction of gold 
ores, and induced him to spread the inquiry over a field 
nearly commensurate with existing mining regions. 

The matter thus collected is rather a painstaking description 
of practice than a discussion of the chemistry and mechanics 
of the stamp-milling process. It will, therefore, it is believed, 
be serviceable to those engaged in the actual work of the 
mill, no less than to the student in the laboratory or lecture- 
room. 

The ordinary operations of the stamp mill do not involve a 
knowledge of nice chemical formulae nor intricate mechanics. 
A good millman has the clarified common sense which lies at 
the basis of all true science, and has won a knowledge of the 
bed-rock principles of this ancient process of gold extraction 
such as no book learning can equal. To him the author has 
talked, and with him he has worked. To him he now humbly 
commends this book with the wish that the millman may 



Vl PBEFACE. 

never do a thing without a reason for it, and always remember 
that where so great improvement has been made much more 
must yet be possible. 

To the student starting out in his career as mining engineer 
or metallurgist, the author trusts the information gathered 
by a comrade, not much his senior, will be serviceable in em- 
phasizing the many-sidedness of the stamp mill, and in 
awakening an intelligent interest in a process whose history 
covers the evolution of a great industry. To those about to 
choose a specialty in metallurgy, a science now covering a 
field so extensive as to be in its completeness beyond the 
mastery of any individual, he suggests gold milling as a 
profitable and fascinating study. 

To the juniors in the profession the author would like to say 
that the gathering of correct information is a work which 
brings its own reward. The publication of data thus collected 
may not be of much moment to this busy world, but it 
clarifies the contributor's ideas and gives a compact crystalline 
form to a quantity of amorphous knowledge, rendering the 
routine work of the profession vastly more productive and 
pleasant. There is no better way of winning knowledge than 
to give away the little that you have ; all that we all of us 
know is but a small matter. The worst of all waste is the 
waste of experience. 

Such have been the feelings that prompted the making of 
this book, and with such the writer offers it now to his fellow 
workers. 

T. A. KiCKARD. 
Denvbb, Col., January, 1897 



TABLE OF CONTENTS. 



PAGS 



Chapter I. — The Philosophy of the Stamp-MujIjINg 

Process. 

The stamp and mortar resemble hammer and anvil — The 
Colorado and California practice viewed from this stand- 
point — The ore like a nut to be broken open — The adjust- 
ment of the mill to this end — The applicability of the pro- 
cess to certain classes of ore — Milling a matter of business. 1-10 

Chapter II.— Gilpin County, Colorado. 

The history of this old mining district — The evolution of 
a peculiar local practice — A description of the work done 
by a representative mill — ^A comparison with other neigh- 
boring plants— The character of the ores treated — Criticism 
of the methods in vogue— The millmen of Black Hawk — 
Suggestions for the future 11-34 

Chapter III.— The Typical Stamp Mills of California. 

Grass Valley in Nevada County — The history of this 
region — Description of the North Star mill and its opera- 
tion — Working costs — The Empire and Idaho mills — Com- 
parisons — Choice of screens — The amalgamating plates and 
their arrangement — Recent changes — The tendency of 
these changes toward a greater degree of battery amalga- 
mation 35-66 

Chapter IV. — Californian Practice in Amador County. 

The early history of Amador — The growth of its mining 
industry— Description of the Gover mill and its operation 
— Working costs — Other mills in this district — Compari- 
sons—The weight of the stamps — ^Changes in the depth of 
discharge — The arrangement of the amalgamating tables — 
This defect of the Californian mills — The general results 
obtained by the Amador mills — The excellence of the work 

done .... 67-74: 

(vil) 



Vlll TABLE OF CONTENTS. 

Chapter V. — The Pbofitable Working of Large Bodies 

OP Low-Grade Ore. 

The Black Hills — Their early development — The lode 
structure of the region — The various stamp mills — Com- 
parisons — The Homestake Company's plant — The rapidity 
of the crushing — Bock-breakers — The regulating of the dis- 
charge by the use of chock-blocks — Screens — The con- 
sumption of mercury 75-86 

Chapter VI. — Milling in the Black Hills, South 

Dakota. 

The arrangement of the Homestake plant — The Highland 
and Dead wood-Terra mills — The order of the drop — The 
concentration of the pyrites — The process of amalgama- 
tion — Incompleteness of the treatment — Shoes and dies — 
The Homestake mortar and its excellence — Working costs 
— Criticism of the arrangement of the amalgamating tables 
— The process as a whole 87-101 

Chapter VII. — Early Australian Methods. 

The first Australian stamp mill — The history of Clunes 
and the Port Phillip and Colonial Gold Mining Co. — The 
arrangement of the old mill — The blanket tables — The 
wells — Barrel treatment of blanket washings — Loss of mer- 
cury — Other mills i^n the district — The South Clunes United 
— The mortar and its arrangement — Double discharge — 
The records of extraction — Character of the ores — Its sim- 
plicity — Suitability of the processes employed — General 
excellence of the milling methods 102-118 

Chapter VIII. — More Modern Australian Methods. 

The first gold discoveries at Ballarat — Leading features 
of representative mills — The Star of the East plant — Its 
arrangement — The old and new mill compared — Incom- 
pleteness of the equipment — The Brittania United mill — 
The use of hot water in the battery — The addition of lime 
to the ore — The New Normanby mill — Comparisons — Ab- 
sence of rock-breakers and feeders — General remarks . . 119-132 

Chapter IX. — Gold Milling at Bendigo, Victoria. 

The story of early discovery— Importance of the district 
— A list of representative mills — The Fortuna crushing 
works — The Pearl mill and its arrangement — The New 
Chum Consolidated — Comparisons — Iron-framed batteries 
— The character of the ore — Criticism of the treatment — 
Absence of labor-saving machinery — Hand-feeding — Gen- 
eral remarks 133-163 



TABLE OF CONTENTS. IX 

Chapter X.— Double-Discharge Mortars in Victoria. 

The Ovens district in Victoria — The Harrietville mill — 
Its arrangement — The peculiar design of the mortar box — 
Double discharge — Comparison between plated and plain 
copper — Treatment of the concentrates — The clean-up — 
Iron-framed battery — The Oriental mill — A simple ore- 
feeder — Pan treatment of tailings — The cost — Other mills 
in the district — Derivation of practice from Clunes — Gen- 
eral remarks 164-166 

Chapter XI.— The Use op the Stamp Mill for Ores 
Unsuited for such Treatment. 

The Thames or Hauraki gold field in New Zealand — Its 
brilliant record — Kepresentative mills — The Saxon and 
Moanataeri mills — Their arrangement — Comparisons — The 
wear of the screens — Complexity of the ores — Their refrac- 
tory character — Inadequacy of the methods of extraction — 
The tailings plants — Suggestions — The use of Muntz metal 
plates — ^A mill test — Confirmation of author's criticisms. . 167-18B 

Chapter XII.— The Stamp Mills of Otago, New 

Zealand. 

The Province of Otago — Its output — ^List of mills — Imita- 
tion of Australian practice — The Phoenix mill — Its arrange- 
ment — The wells and blankets — Barrel treatment — The 
screens — Character of the ores — The process versus plate 
amalgamation — Tests — The concentration of the pyrites — 
Incomplete equipment — The process viewed as a whole . 184-195 

Chapter XIII.— A Review of Australian Practice. 

Wet processes versus dry — The simple nature of most 
Australian gold ores — The refractory ores of Queensland 
and New South Wales — A comparative table — Clunes, 
Bendigo, and Ballarat — Similarity of the mills — The con- 
trast between Otago and the Thames, New Zealand — Treat- 
ing ores unsuited to the stamp mill — Charters Towers in 
Queensland — Chlorination of concentrates — General sur- 
vey 196-204r 

Chapter XIV.— The Wear and Tear of a Mill. 

Shoes and dies— Their time of service — ^A comparative 
table — Cost — Varieties of material used in different regions 
— Colorado, California, Arizona, Australia, and New Zea- 
land—The use of rock-breakers diminishes wear and tear 
— Results of general experience 205-215 



X TABLE OP CONTENTS. 

Chapter XV.— The Fix)uring of Mercury. 

The terms " sickening " and *' flouring" — Carelessness in 
their usage — Definitions — Flouring not necessarily injuri- 
ous — How caused — The sickening of mercury and the 
cause of it — The use of condenser water — Subdivision of 
mercury in patent pulverizers — Remedies 216-219 

Chapter XVI.— Comparisons. 

The actual cost of milling in various districts — Figures — 
Labor and machinery — Comprehensive table of treatment 
— Weight of stamps — Iron-framed batteries — Depth of dis- 
charge — Kate of crushing — Wire screens — The character of 
the ores — Use of the stamps in concentrating mills — Need 
of sizing apparatus — Supplementary leaching processes — 
The wide field covered by stamp milling 220-235 

Chapter XVII.— Mills and Millmen. 

Need of technical knowledge in milling — Gradual de- 
velopment of gold- winning processes — Arrangement of the 
amalgamating table — Lack of accurate data of work done 
in mills — Waste of experience — Rarity of careful investiga- 
tion of methods and appliances — Disinclination of millmen 
to experiment 236-243 

Chapter XVIII. — The Future of the Stamp Mill. 

Failures in the design of stamp mills and their cause — 
An example in Idaho — Comparison between stamp-milling 
and other processes — Battery amalgamation — Failure of 
steam stamps in amalgamation — Relation between pulveri- 
zation and amalgamation in the stamp mill — New con- 
trivances proposed to replace the stamp mill, and their 
results — Conclusion 244-249 

Glossary op Stamp-Milling Terms 260-153 



LIST OF ILLUSTRATIONS. 



PAGE. 

Central City and Black Hawk, Colo 13 

Interior of the Hidden Treasure Mill, Black Hawk, Colo. ... 19 

Gilpin County bumping table 30 

Type of a California ten-stamp mill 38 

Mortar used at the North Star mill, Grass Valley, Cal 41 

Battery screen frame 42 

Amalgamating tables at the North Star mill. Grass Valley, Cal. 43 

North Star mill, Grass Valley, Cal 47 

North Star mill. Grass Valley, Cal 48 

The Union mill, San Andreas, Cal 51 

Impaired wire screen 53 

Interior of the Gover mill, Amador County, Cal 59 

Sketches showing arrangement of amalgamating tables .... 63 

Mortar for an Amador stamp mill 68 

Section of an Amador stamp mill 73 

Deadwood-Terra Company's open cut. South Dakota 77 

Interior view of Golden Star mill, Homestake Mg. Co., South 

Dakota 85 

The Homestake Company's stamp mills. South Dakota 88 

Longitudinal and cross-sections of the Highland mill. South 

Dakota 88 

Plans of the Homestake mortar, Homestake Mining Co., South 

Dakota 94 

The stamp mill of the South Clunes United Co. , Clunes, Victoria 107 

Amalgamating barrel 110 

Crushing mill at Ballarat, Victoria 120 

An iron-framed battery as erected at Bendigo, Victoria .... 149 

Details of battery at Bendigo, Victoria 150 

Double-discharge mortar used at the Harrietville mill, Victoria, 156 

Harrietville mill, the Ovens District, Victoria 161 

Mills in the Thames district. New Zealand 180 

An Arizona stamp mill 210 

The Elmore mill, Kocky Bar, Idaho 232 

Plan of forty-stamp gold mill, Aztec Gold M. and M. Co. . . . 238 



CHAPTER L 
The Philosophy op the Stamp-Milling Process. 

Milling is a process of ore reduction whereby the extraction 
of the valuable metals is effected at a minimum of expense. 
Gold stamp milling is that particular process in which a heavy 
cylindrical body of iron is made to fall upon the ore in such 
a manner as to crush it, and thereby facilitate a separation 
between the gold and the valueless minerals by which the 
gold is encased. The latter weigh less than the former, and 
are removed by the aid of water. The gold is then collected 
through the agency of mercury with which it readily forms 
an alloy or amalgam. From this combination it is finally ex- 
tracted by the distillation or retorting of the mercury. 

The mechanism of the stamp acts on principles similar to 
those underlying the crudest devices used by man. It may be 
likened to a hammer, of which the shoe is the hammer head, 
the stamp stem is the handle, and the die the anvil. The ore 
itself has been compared to a nut struck by a hammer whose 
blow has separated the valueless shell (the quartz) from the 
valuable kernel (the gold). Similes such as these may be carried 
further. The hammer falls, the anvil is fixed ; the same rela- 
tion exists between the stamp and the die inside the mortar. 
The anvil is made of softer metal than the hammer, so also 
the die is often, and should be always, of iron or steel less 
hard and more tough than that composing the shoe. The 
movement of the hammer and the drop of the stamp are both 
intermittent. There is in both a loss of power due to inelastic 
impact ; in both, also, there is an apparent loss of time due to 
the interval separating the blows. In the case of the hammer 
that interval of time is utilized in the shifting of the material 
which the hammer blows are shaping; in the case of the 



2 BTAMP MILLING OF GOLD ORES. 

stamp the time is turned to advantage in the opportunity 
which it gives for the water to assist in the separation of the 
gold from the quartz. 

The hammer falls upon the dry and wide surface of an 
anvil, the stamp, however, drops upon a face of iron (the die) 
confined within a narrow box (the mortar). Water covers 
the die and the ore lying upon it. The blow of the falling 
stamp not only crushes the ore, but also causes a violent 
pulsation of the water. That pulsation becomes converted 
into an irregular splash against the sides of the mortar. The 
latter has an opening in front, through which the water is 
discharged, carrying with it the crushed ore. This, called the 
pulp, spreads itself over tables placed on an incline, which 
are lined with a metal, usually copper, having an amalga- 
mated surface such as will arrest the particles of gold and at 
the same time permit the grains of quartz and other valueless 
material to pass over it and out of the mill. 

The work done by the hammer is dependent upon its 
weight, the rapidity of its blow, and the distance through 
which it strikes. In stamp milling this simple analogy has 
many departures from it. These can best be explained by 
examining two extreme types, such, for instance, as are 
afforded by the divergent methods of Colorado and California. 

If you go to Black Hawk, in Gilpin County, Colorado, and 
walk up the long straggling street, you will hear the stamps 
falling so slowly that the drop of individual ones can be dis- 
tinguished. At one time the rate used to be about 24 per 
minute, now the speed is regulated at an average of 30 drops 
per minute. If, on the other hand, you should visit Angel's 
Camp, in Calaveras, or Sutter Creek, in Amador County, Cal- 
ifornia, you would hear the muffled roar of stamps falling so 
fast as to make it quite impossible to recognize the drop of 
any single one. There the rate would average 90, with an 
occasional maximum of 105 drops per minute. 

Upon farther inquiry we shall find that in Colorado the 
stamp weighs 500 to 600 pounds, the most common figure at 
Black Hawk being 550. In California the extremes are greater, 
from 750 to 1000 pounds, with a frequent repetition of 850. 
In Colorado the drop ranges from 16 to 20 inches, in Cali- 
fornia from 4 to 6 inches. If we now multiply these three 



PHILOSOPHY OF THE STAMP-MILLING PROCESS. 3 

factors of weight, drop and speed, we find that the theoretical 
work done is nearly equal and is approximately 1 horse- 
power. But the results aimed at in stamp milling cannot be 
measured in foot-pounds. The efficiency is gauged rather by 
the amount of ore crushed and the completeness wherewith 
the gold is extracted. The stamp mill is both a crushing 
machine and an amalgamating contrivance. Therefore, not- 
withstanding the approximate equality in the sum total of 
theoretical work, the Colorado mill crushes only one ton per 
24 hours ; while in California, with an ore of similar hardness, 
the amount is from two and one-half to three times as much. 
Why is this difference ? Should it exist ? Is it the result of 
Ignorance or the outcome of experience ? In answering these 
questions we shall have to discuss the bed-rock principles of 
stamp milling. 

To the first question the answer comes promptly. The 
difference is due to the dissimilarity of the ores treated. That 
which goes to the mills at Black Hawk contains an average 
of 15 per cent, of pyrite. The gold which it carries is in a 
very fine state of subdivision, and is intimately associated 
with the pyrite. On the other hand, the mill stuff treated at 
Angel's Camp, or Sutter Creek, carries only from 1 per cent, 
to 2 per cent, of pyrite, and the accompanying gold is not 
nearly so intimately mingled with it, but rather occurs in 
clean quartz and in a comparatively coarse condition. The 
immediate result is that coarse crushing, which is, of course, 
more rapid and therefore more economical than fine crushing, 
will do for the California simple quartz ores what it can not 
do for Colorado's complex pyritic material. 

Let us return for a moment to the simile of the hammer 
and the nut. The hammer which cracks open the nut may 
liberate the kernel without crushing it. I have seen gold 
bearing quartz which could be likened to such a nut. The 
gold occurred in cavities in quartz of a honeycombed kind. 
In dealing with such an ore there is just a certain blow which 
will break the brittle quartz and liberate the ductile gold. 
Such conditions are ideal. Usually the hammer will not only 
break the shell but also crush the kernel, so that the particles 
of both become confused together. This more nearly resembles 
the ordinary action of the stamp, which has, moreover, to deal 



4 STAMP MILLING OP GOLD ORES. 

with a material in which the gold and its envelopment of 
gangue, the valuable and the valueless constituents, are so 
irregular in size and so intermixed that the one is often 
crushed too much and the other too little. 

Particles of gold will remain attached to or enveloped by 
pieces of quartz, rock, or pyrite, unless crushed to a fineness 
such as compels a separation. Without such a complete 
separation amalgamation will not take place. It is obvious, 
therefore, that when the gold in an ore is coarse the process 
of crushing need not be carried so far as when the gold is in 
a state of minute subdivision.* Again, it is evident that the 
longer a particle of ore remains inside the mortar the more 
frequently, other things being equal, will it undergo the blow 
of the stamp, and that the more often it is struck by the stamp 
the finer the condition into which it will be pulverized. 

How are these factors regulated? In two ways, by the 
depth of discharge and by the mesh of the screen. 
• The ore upon the die is under water. After it has been 
sufficiently crushed it is ejected by the splash of the water 
through the screen or grating which covers the opening in 
the front of the mortar. The surface of the water is approxi- 
mately level with the bottom of the aperture occupied by the 
screen. In Colorado the depth of discharge (also termed the 
issue), as measured by the distance from the bottom of the 
screen to the top of the die, is 14 inches. In California it is 
4 inches only. Here we have found one of the reasons for 
the diffeience in the crushing capacity of the mills. Though 
the same amount of power be expended in lifting the stamps, 
and though the screen used be of a similar mesh, yet in 
Colorado less ore is crushed in a given time because the 
stamp falls through 10 inches more of water and has to eject 
the crushed ore at a level 10 inches higher than in the 
California mill. The greater thickness of the cushion of 
water deadens the blow of the stamp and weakens the force 
of the splash. 

What is the result upon the ore ? It is found that even with 
screens of equal mesh tho crushed ore remains inside the deep 

*Only microscopio examination can give one an adequate idea of the minuteness 
of the particles of gold in certain ores, or of the extent of its diffusion through the 
mass. 



PHILOSOPHY OF THE STAMP-MILLING PROCESS. 5 

Colorado mortar longer than in the shallow one used in Cali- 
fornia, and that, therefore, the pulp discharged from the 
former has a fineness much greater than that delivered from 
the latter. The screen is a controlling, but by no means final, 
factor in regulating the rapidity of the discharge and the size 
of the particles of the pulp. In California 30-mesh wire cloth 
is ordinarily employed; in Colorado, slot-punched screens 
equivalent to 50 mesh. The one represents 900 and the other 
2,500 holes per square inch. In neither case, however, is the 
ore crushed just to that particular size permitting of exit 
through the screen, but much of it is crushed to an unneces- 
sary and far greater degree of fineness. It is found with the 
use of screens of similar mesh, say 40, that in a California 
battery about 50 per cent., and in a Colorado mill fully 70 
per cent., of the pulp will pass through a 100-mesh sieve. This 
is due to the pause which occurs between the successive drops. 
Particles of ore which have been pulverized to a size which 
would allow of their discharge through the screen are enabled 
to fall back and are recrushed by the succeeding blows of the 
stamp. In the fast (California) mill the interval between 
the drops is two-thirds of a second; in the slow (Colorado) 
mill it is two seconds. 

The irregular splash of the water inside the mortar causes 
an unequal sizing of the pulp. The particles of pulverized 
ore strike the screen in a haphazard way. It is a question of 
hit or miss whether any particular particle be thrown against 
an opening or a blank. If it fail to pass through, it is drawn 
back by the recession of the water and undergoes further 
pulverization. This action must obviously take place to a 
greater extent in a deep mortar with a punched screen which 
offers less opportunity for free discharge than in a shallow 
mortar with a wire screen which offers increased facility for 
the issue of the pulp. Hence the tendency to a greater degree 
of excessive pulverization in the Colorado mill as compared 
to the Californian. 

This excessive pulverization is desired by the Colorado 
millman because it enables him to break the close intimacy 
between the finely divided gold and its associated pyrite. He 
therefore has combined the features of a long and slow drop 
with a deep discharge. The comparatively light weight of 



6 STAMP MILLING OF GOLD OkEj?. 

the stamp is necessitated by the excessive height of the drop. 
He has also designed a mortar which is wide and roomy. 
This causes the splash of the water inside the battery to be 
weak, and compels the pulp to remain inside until pulverized 
to a fineness much exceeding that required merely for its 
passage through the screen openings. Then, having obtained 
conditions enabling him to separate the gold from the pyrite 
in the ore, he catches it upon amalgamated copper plates 
which are arranged along the front and back of the mortar. 
To sum up, he makes the crushing feature of his mill sub- 
servient to amalgamation. Now turn to the Californian. 
His ore does not require such fine pulverization, because it 
does not present such difl5culties to the separation of the gold 
which it contains. He finds he can readily save the gold in 
the pulp, after it has been ejected from the battery, by catch- 
ing it upon tables covered with amalgamated sheets of copper. 
He, therefore, has made his battery primarily a fast-crushing 
machine, and only incidentally an amalgamator. If gold is 
caught within the mortar, as some usually is, so much the 
better ; but, I insist, amalgamation inside the battery is sec- 
ondary to the obtaining of conditions facilitating rapid crush- 
ing. The Californian has consequently found that for his 
purpose the best arrangement is that of a stamp mill having 
heavy, fast dropping stamps which will induce rapid pulveri- 
zation ; he has designed a narrow mortar, thereby diminish- 
ing the opportunities for the resettling of particles of pulver- 
ized ore ; he has utilized a shallow discharge, thereby increas- 
ing the force of the splash of the water and accelerating the 
exit of the pulp through the screen. In this case the intro- 
duction of plates for amalgamation is rarely admissible, 
because in mortars having so shallow a discharge as 4 inches, 
the violent agitation of the pulp abrades or scours the surface 
of such plates. 

We have given the reason for the marked difference existing 
between the milling methods of two districts in the same 
country. Next comes the query, Should such a difference 
exist ? Yes, if stamp milling is to be the process employed in 
the reduction of the ore. The first axiom of successful milling 
is that the process which is the best adapted to the ore and 
the cheapest under any given conditions is the one which 



PHILOSOPHY OP THE STAMP-MILLING PROCESS. 7 

should be chosen. It has been claimed by many that the 
stamp mill is not the best machine, nor amalgamation the 
best process, for the treatment of the ores of Gilpin County, 
the home of the Colorado method which has been described. 
It has been stated that the practice which is in vogue there is 
incorrect from a metallurgical standpoint, and out of date. 
Concentration has been suggested as the proper process. The 
examination of this question is an excellent example of the 
relation existing between the commercial and technical sides 
of metallurgical practice. 

Take, for instance, the case of 10 tons of ore, each carrying 
$8 worth of gold. The yield by amalgamation would be, 
say 65 per cent., equivalent to $52, to be supplemented bv a 
further yield of 15 per cent., or $12, in the form of one 
ton of pyrites concentrates. There would thus be obtained 
$64 out of the original contents ($80) of the ore, equal 
to an extraction of 80 per cent. The ton of concentrates 
would be shipped to Denver, where the smelter would pay 
95 per cent, of the assay value, less the charge for treatment, 
$5, and freight, $1.50. The net return would be $9.90. This 
last figure added to the gold obtained by amalgamation, 
which is usually sold to a local bank, gives $56.90 as the final 
return from the 10 tons of ore. 

Now, suppose concentration to be substituted for the pres- 
ent method of milling. The 10 tons of millstuff would be 
concentrated into one. Supposing the extraction to be 80 
per cent., then the one ton of concentrates would have an 
assay value of $64. When forwarded to the smelter this 
material would be required to pay a higher treatment charge 
because of its higher value and its greater silica contents. 
The charge would be from $8 to $10 per ton. Let us take the 
lower figure, $8, and add to it the freight, which would be $2, 
or 50 cents more than that levied on the lower grade material. 
Add $1 for sampling,* and the total deductions, the smelter 
paying 95 per cent, of the assay value, foot up to $14.20, 
leaving $49.80 as the final return. There is, therefore, a 
balance of $7.10 in favor of the present method. 

In the above example the costs of milling and concentration 

♦Avoided in the case of the very low grade, finely pulverized and homogeneous 
stamp-mill concentrates which are hand-sampled by means of a tube resembling 
a cheese-taster. 



8 STAMP MILLING OF GOLD ORBS. 

have been supposed equal, and an extraction of 80 per cent, 
allowed in both cases. There are some ores in Gilpin County 
which, being more heavily charged with pyrites, galena, etc., 
are more suitable for concentration than stamp milling; 
similarly there are others so poor in pyrite and so free milling 
as to put concentration out of court. On the whole, however, 
the above-cited example will be suflScient to indicate how 
largely local commercial considerations enter into the choice 
of the method of ore reduction. 

The last question which we set out to answer was whether 
the existing practice in Colorado and California is the result 
of ignorance or the outcome of experience. This has already 
been answered in the foregoing paragraphs. Nevertheless, a 
few additional notes may prove instructive. 

In both cases, California and Colorado, the first methods 
adopted were derived from a common origin. They were 
brought westward by the pioneers who came from Georgia, 
and who had themselves in turn borrowed their ideas from 
the classic regions of Transylvania. In both cases the evolu- 
tionary changes of a local experience have gradually so 
modified the original method that the common parentage of 
the local practice is no longer evident to the investigator. 
On the Pacific coast the ore was found to be even more 
simple in character than that of Georgia ; the methods first 
introduced were, therefore, modified in such a way as to in- 
crease the relative importance of the crushing capabilities of 
the stamp mill. The experience of the placer miner was 
allowed to influence the choice of the apparatus intended for 
the process of amalgamation. As a consequence the chem- 
ical side of the process has remained practically unchanged, 
while the crushing mechanism has been improved by a num- 
ber of ingenious devices giving greater automatism in the 
handling of the ore. 

In the Rocky Mountains of Colorado it was found that the 
mills erected by Gregory and the other Georgian pioneers did 
good work upon the simple surface quartz which was first 
discovered, but so soon as the unoxidized heavily pyritic ores 
of the deeper workings of the mines were encountered the 
extraction fell off woefully. The district was paralyzed. The 
smelter, with its process of fire reduction, came to the rescue. 



PHILOSOPHY OP THE STAMP-MILLING PROCESS. 9 

Stamp milling became supplemented by smelting. At the 
same time battery amalgamation was found to be facilitated 
by deepening the discharge and widening the mortar. This 
was necessarily accompanied by a higher drop, which in turn 
prevented the use of very heavy stamps. Gradually the 
•everyday experience of the millmen enabled them to arrive 
at the conditions most favorable to good Work. The smelter 
also realized the value, for fluxing purposes, of the low-grade 
iron pyrites composing the mill concentrates, and so the prac- 
tice of the camp was whittled into its present shape. 

While, therefore, in Gilpin County a quite peculiar set of 
circumstances renders an apparently antiquated method of 
milling well adapted for the economical treatment of the ores 
of that particular district, nevertheless the unthinking imita- 
tion of this practice in other districts, working under dis- 
similar surroundings, is not to be commended. Elsewhere in 
the State of Colorado the Californian method has been found 
better suited to the cheap reduction of the ores, and it has 
been successfully adopted. On the other hand, conditions in 
certain parts of Montana and Arizona are such as occasionally 
to render slow, fine crushing, accompanied by battery amal- 
gamation,* more desirable than either fast, coarse crushing 
without inside amalgamationf or straight out-and-out con- 
centration. Science is only organized common sense ; the 
most truly scientific method is necessarily the most sensible. 
It is the business of the millman to determine what are the 
exact conditions and then adapt his method to them. In 
California there are, for similar reasons, certain variations 
from the typical local method, such as indicate an approach 
toward the old Colorado practice. Thus, in Amador County, 
the depth of discharge has been deepened to seven or eight 
inches and an amalgamating copper plate is attached to the 
front inside of the mortar. The result is to lessen the rate of 
crushing, increase the fineness of pulverization, and augment 
the proportion of gold saving effected within the battery. 

Milling is a matter of business. The closest application of 
technical science may occasionally be obtained at the expense 
of the best commercial results. Thus you may save 80 per 

*Plus tables and supplementary concentration. 

-hBut with amalgamating tables followed by Frue vanners. 



10 STAMP MILLING OF GOLD ORES. 

cent, of the value in an ore at a cost of 80 cents per ton. By 
the use of further skill and added apparatus you may extract 
five per cent, more, but the value of that additional saving 
may be less than the extra expense, so that while you get 
$3.20 out of a $4 ore at a cost of 80 cents, you obtain $3.40 at 
a cost of over $1.10. Your additional 20 cents' worth of gold 
has cost more than 30 cents to obtain. 

Metallurgy is an applied art. When employed in the prac- 
tice of milling it should mean an extraction as large as ia 
consistent with the least expense. Chemistry must be supple- 
mented by common sense and mechanics by business insight. 
It is this effective combination which has given the westera 
mining industry its prosperity and progressiveness. 



CHAPTER II. 
Gilpin County, Colorado. 

Gilpin County, the most important gold mining-district of 
the State of Colorado, lies at the foot of the main range of 
the Rocky Mountains. With its record is interwoven the 
beginning of the history of Colorado and the birth of a great 
industry. 

In the days when this part of the United States was yet a 
portion of Kansas, an unknown region overrun by the Indian 
and the now almost extinct buffalo, a motley crew of eager 
seekers after gold were drawn thither by the fame of Pike's 
Peak. The pioneers of 1858 were mostly Georgia men, some of 
whom had been to California. When their El Dorado proved 
a delusion, the more enterprising, leaving the log cabins 
by the side of the River Platte — log cabins which marked the 
site of the now stately city of Denver — followed the course of 
Clear Creek up the winding canons and found the river gravel 
which produced the first output of gold. The alluvial de- 
posits, however, owing to the narrow, rocky channel and the 
rapid current of the stream, were of but small extent, and 
the area available soon becoming exhausted necessitated the 
search for further gold-bearing ground. It was then that the 
pioneer, following the rapidly narrowing beds of the moun- 
tain torrents, found himself fronted by the ramparts of the 
mighty Rockies themselves, and turning to one side discov- 
ered in the quartz lodes the original source of the river gold. 

In April, 1859, John H. Gregory first made his way up North 
Clear Creek and found good prospects near Black Hawk. On 
May 6 the Gregory lode was discovered. The fame of Gregory 
Diggings at once drew to it all the wandering population 
scattered among the neighboring hills. Other lodes were dis- 
covered in rapid succession. Then there commenced the 

(11) 



12 STAMP MILLING OF GOLD ORES. 

active working of the gold veins, which is the only excuse for 
the existence of Black Hawk, Central City, and Nevadaville ; 
which for a long time made Gilpin County the chief gold 
producer of Colorado ; which trained the men who opened up 
the Leadville mines, and gave the money to those who built 
up Denver. 

While the area of Gilpin County is only 122 square miles, 
its output to date is estimated at $76,000,000. Its largest 
annual production (in 1889) was $3,334,300, while that for last 
year is estimated at $2,000,000. 

As a milling center it ranks among the most important. 
The history of the evolution of the milling practice of Gilpin 
County forms one of the most interesting chapters in the 
record of the American mining industry. Briefly it was thus : 
The first machine introduced was the arastra, which at first 
proved satisfactory, but was soon found to be too slow for the 
American, however well suited to the Mexican. Stamp mills 
of three, four, and six heads were erected by the Georgian 
miners, and these in turn gave place to larger plants modeled 
after the California fashion. This type — fast drop and shallow 
discharge — of battery was adapted to the treatment of the 
surface quartz. All went well ; the output and importance 
of the district steadily increased. In July, 1860, sixty mills 
were at work in the county. The gold saving at that time 
was all done by riffles, carrying quicksilver, but in the follow- 
ing year the first copper plates were introduced. Soon after 
this the camp received its first check ; the oxidized material 
of the upper portions of the lodes began to give place to ore 
which was less quartzose, which contained more of the coun- 
try rock as vein filling and carried a percentage of pyrites 
which steadily increased with depth. This was at levels vary- 
ing from 100 to 200 feet. The mills which had previously 
been extracting from 60 per cent, to 75 per cent, of the gold 
contents, gradually commenced to return only 50 per cent., 40 
per cent., and then 30 per cent. None but the richest ore 
would now pay ; the mills swallowed up two-thirds of the 
yield which should have rewarded the miner's toil ; some of 
the mines were forced to shut down, while others had to con- 
fine their development to the narrower, richer portions of the 
lodes. Gilpin County as a mining field seemed to be about 



14 8TAMP MILLINa OF GOLD ORES. 

to write " finis " across the portals of its mills and engine- 
houses. At this juncture a small smelting establishment was 
erected in the district, and the metallurgist came to the rescue 
of the baffled millman. It was in the spring of 1867 when 
the Boston & Colorado Smelting Works were first established 
at Black Hawk. The Swansea process of copper smelting was 
introduced, the matte being shipped East. In the year which 
followed most of the mills remained idle ; many of the mines 
were shut down, for only those could afford to be worked 
which yielded ore sufficiently rich to meet the cost of smelt- 
ing. For some years the smelter tcok the place of the stamp 
mill, but in the interval the energetic, resourceful men of the 
place studied the successful treatment of their pyritic ores, 
and after experiments, which cost much time and more 
money, eventually in the beginning of the "seventies" they 
solved one of the knottiest questions ever put to the miner. 

We are now familiar with the terms " free milling" and 
*' refractory" ore, and we are to some extent cognizmt of the 
different treatment required by the two types, but such 
knowledge as we possess is in no small measure due to 
the plucky manner in which the millmen of that day over- 
came the obstacles presented by the treatment of a most 
difficult ore. 

The accompanying tabulated statement illustrates how, from 
the Californian or " fast drop, shallow discharge" type of 
milling practice, Gilpin County has arrived at a distinct type 
which may be summarized in contrast as the "slow drop, 
deep discharge" system. 

The figures herewith given will serve as a text for the 
paragraphs which follow. 

One of the best mills of the district is the Hidden Treasure, 
the property of the California mine, and as it thoroughly rep- 
resents the best practice of Gilpin County, I shall take it as 
a type and endeavor to describe fully the methods of work. 

The Hidden Treasure plant consists of 75 stamps in three 
sections of equal number ; of these two are of an older date than 
the third. The stamps of the former are supplied with screw 
tappets, while in the case of the latter the tappets are kept 
in place by means of gibs and keys. The last-mentioned 
method is much preferred. 



GILPIN COUNTY, COLORADO. 



15 



The stamps weigh 550 pounds each and fall at the 
from 80 to 32 drops per minute. The order of drop is 1-5-2-4-3. 



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Si. 



16 STAMP HILUNa OF GOLD OBES. 






i 



I 



/ '^ ^ The height of the drop varies from 16 to 18 inches. The issue 
or depth of discharge — ^that is, the distance from the top of 
the die to the bottom of the screen — is 13 inches when new 

i^f/j / ;' , dies have just been placed in position, and increases to a 
• • / f maximum of 15 and 15^ inches as they wear down. 
'^ '^f^^. The shoes are 5^ inches deep and 8 inches in diameter. The 
dies are plain, cylindrical in shape and fit into a round seat 
in the mortar bed. They are 3^ inches deep, slightly wider 
than the shoes, and are kept in place by sand which is packed 
tightly between and around them. The shoes weigh from 
83 to 86 pounds each, the dies from 46 to 48 pounds; both 
are made of cast iron at the local foundries. The wear of the 
shoe is at the rate of 11.3 ounces of iron per ton of ore crushed, 

!^^ ^^ , that of the dies is 4.5 ounces per ton. 

' The capacity of the mill varies somewhat. At present 60 

heads are engaging in treating custom* ore, and these crush 
faster than the 25 which are fed by millstuif coming from the 
California mine. The entire mill crushes 320 cords or 2,560 
tons per month of 30 days. The mills do not close down on 
Sundays. From January to May, inclusive, 1,066.48 cords were 
crushed by the 50 heads. This, taking a cord as equal to 
8 tons, comes to 1.14 tons per stamp per 24 hours.f 
The screen used is that known as burr slot ; the slots are 
"^U^ horizontal and alternate. No. 1^ is generally employed, that 
size being equal to a 50-mesh wire screen. The screen surface 
is 4i feet by 8 inches. During the past year 200 feet of 
screens were used up, or at the rate of 66.6 screens per year. 
The average life of a screen was therefore 81 days. With the 
ore coming from the California mine they last three months, 
it being the custom to turn the screen upside down as soon 
as the lower portion, the first affected, gets worn. Occasion- 
ally, after having served for the treatment of the company's 
ore, the screens are used in the custom sections of the mill 

* Custom milling is a great feature of tlie mills of this section. The charges are 
17.50 per day per battery of five heads, this to include the concentration of the 
pyrites and their shipment on board the railway car. For small lots the rates are $15 
per cord for milling and $2 per cord for concentrating. The last charge varies from 
|1 to 18, according to the percentage of pyrites in the ore. 

+ In this district there is a curious custom of measuring ore by the " cord," a unit 
derived Irom firewood measurement, and equal to 8x4x4 feet or 128 cubic feet. A 
cord of mill ore is equal to 7K to 8 tons, and one of smelting ore to from 9 to 10 tons 



S 



(!•, 



GILPIN COUNTY, COLORADO. 17 

where coarser crushing is required by an ore coming from a 
shallower level and of a somewhat different character. 

The average of a year's work shows that the ore yields con- ^. 
centrates at the rate of 13 per cent., and of a mean net value ^^{y ctf^ 
oi $15 per ton. Both the quantity and quality of the concen- ^ 

trates vary directly with the richness of the millstuff. , /j 

In retorting, the percentage of bullion yielded by the amalgam ^^U/^.^,^. 
varies from 30 to nearly 50 per cent., but 40 per cent, may be 
considered the average. The bullion contains 782 to 786 per 
thousand of gold and 207 to 211 of silver. 

Six and a half tanks of quicksilver are consumed by the 75 %/. 
stamps in one year. This amounts to 4.3 dwts. per ton of ore ^ ^'^V 
crushed. The quantity of water used in the mill is at the ^^^^^/;, 
rate of 2 gallons per stamp per minute. ^*a/~ 

The gold saving is done in the mortar boxes, on amalgamat- aJ^ 
ing tables, by the blankets, and finally by concentrators. The ^^^^^^ii^ 
last mentioned were formerly supplemented by buddies or l 
"tyes," but these have now been discarded. The mortar box ^/ 
itself does most of the work of arresting the gold. This is ^ <! 



effected by the free mercury which is added, supplemented fX ^ 
by two amalgamating plates arranged along the front and ^^"^^ 
back of the mortar. They are both made of plain copper. 
The back plate is 12 inches wide and 4i^ feet long. The front 
plate is of the same length, but has a width of only 6 inches. 
The two plates are arranged differently, that at the back 
being placed at an angle of 40 degrees, while the front one is 
nearly upright. At the front of the battery and above the 
screen frame there is an opening ordinarily covered by 
canvas, by the lifting of which the millman is able to intro- /7 / i ' 
duce his arm and can tell by the feel of the front plate ^^t[ ' 
whether the correct quantity of mercury is being added by >.. ^^ 
the feeder. The regulation of the addition of mercury is ^^^^lu 

thus effected without the stoppage of the battery or the ^^ 

removal of the screen frame. Upon an average the feeder 
adds a half thimbleful of quicksilver every hour. As a test 
it was found that in crushing one cord (8 tons) of ore carry- 
ing gold at the rate of half an ounce per ton, there were added 
4^ ounces of mercury, one drop as large as a medium-sized pea 
every hour after the first six hours. v 

The amalgamating tables are of copper and are in one / /« -. 



^ 



18 STAMP MILLING OP GOLD ORES. 

length of 12 feet, having a breadth of 4 feet. They slope 2^ 
inches per foot. In the crushing of three cords* of half ounce 
stuff (10 dwts. per ton) it was found that the one copper 
table used required 5 ounces of mercury to dress it, while 
there were used for the dressing of the front inside plate 3 
ounces, and for the back or wide plate 4 ounces. 
The blanket strakes, or "blanket strips," are 3 feet long 
'^ ^Xi ^ K^ ^^^ ^^ inches wide. They are washed three times per day or 
'"^^ every four hours, an interval which, with rich ore, is de- 
creased sometimes to two hours. They serve to arrest any 
escaping amalgam or mercury, as well as "rusty " gold and 
the heaviest pyrites, together with particles of ore to which 
gold is still attached. They may be perhaps needed to save 
the last-mentioned class of material, but for the rest the mill- 
man is probably correct in his belief that they could be 
discarded without any loss, since this work is done by the 
concentrating machines. 



^^Ca K k^ From the blankets the pulp passes to the concentrators, 

^Hc^ which are shaking tables, called in the locality, where they 

are also constructed, "bumpers." They are the outcome of 

Gilpin County practice, and are a variation of the Rittinger 

type; see illustration. In the Hidden Treasure mill there 

are five of these, each subdivided into two parts. The new 

model is of a lighter pattern than the old, which was divided 

into three partitions. The speed is regulated according to 

the percentage of pyrites in the ore, but will average at the 

rate of 130 strokes per minute. 

^^^Va f ^^ ^^^ total amalgam obtained, two-thirds is the yield of 

> Avuy the plates inside the mortar box, the amount being fairly 

/^a'JL divided between the two.f On cleaning up, the sand found 

in the battery around the dies is not panned but returned to 

the mortar on restarting. The outer amalgamating tables are 

^^A,tv_ xy. cleaned up every 24 hours, but the inside plates only every 48 

I hours. With poor ore the last-mentioned period is further 

prolonged. 

* A cord is the unit of firewood measurement. It consists of 128 cubic feet, 
equivalent, in the case of broken millstuff, to 8 tons. 

+ It is found, therefore, that the direct saving of the gold (the saving in the 
blanketings and concentrates is indirect) is evenly divided between the three 
amalgamating appliances— the front inside battery plate, the back inside plate, 
and the outside table. 




■^ 



20 STAMP MILLING OF GOLD ORBS. 

At the general clean-up the amalgam, as it comes from the 
plates, is placed in a mortar to be ground, with the addition of 
hot water, until of even consistency. The now dirty water is 
decanted and mercury is added until the amalgam is thin. The 
thin-flowing amalgam (or, rather, mercury, which has absorbed 
the amalgam) is decanted from one porcelain dish to another 
several times. As the pyrites, dirt, etc., rise to the surface 
they are skimmed off by the operator's hand. The clean 
amalgam which finally remains is then squeezed through 
canvas. The skimmings obtained are reintroduced into a 
mortar and treated separately, being ground with th^ addition 
of hot water and fresh mercury. When fairly clean, a bit or 
two of potassium cyanide is added to render the mercury 
more active by the decomposition of the oxides injurious to it. 
'Piy- In retorting, the interior of the iron retort is either chalked 

^'■'^ 'n^^ or lined with thin paper. The latter method is to be pre- 
ferred. The balls of dry-squeezed amalgam are put into the 
retort, broken with an iron rod, and then pressed down until 
hard and uniform. A good-sized bolt with a nut at the end is 
a good tool for this purpose. The cover is then put on and 
luted down with clay. 

The only chemical used in the mill is potassium cyanide. 
CUvJL Of this 26 cans of 10 pounds each were used in a year, during 
which time 28,793 tons of ore were crushed. The tables are 
dressed every 12 hours with a weak solution of 2 ounces of 
cyanide in three gallons of water. The tendency in Gilpin 
County, as in California, is to diminish the use of nostrums 
^ in the gold mills.* 

- A. ^jJh^ The tables are brushed twice per day with a mop, mercury 

^ ^ being added if the amalgam is found to be too hard, that is,. 

^^^ too dry. 

The ore is delivered by a steam tramway and discharged 
^-<A^ into ore bins. There are no rock-breakers. The feeding is 

i^ /I ' done by hand. There is one feeder for every 25 stamps, the 
pay being $3 per shift. The power for driving the machinery 
is derived from water and steam. There is an overshot water 
wheel of 50 feet diameter and 4| feet breast. For four 

♦The danger of the use of chemicals arises from the very Imperfect knowledge 
which the millman usually has as to the reactions induced. In the above case, for 
instance, we know potassium cyanide to be a solvent of gold, and it can therefore 
readily be seen how very careful should be the use made of it. 



a 



\u^)^ 



1 



GILPIN COUNTY, COLORADO. 21 

months the water power suffices, for four months (in the ^ 
severe winter) the mill is worked entirely by steam-power, ^ife^ 
and for the remaining four months the two motive powers . , 
are combined. Firewood costs $4.75 per cord, delivered. The ^^^ 
cost of milling in 1890 was at the rate of 84 cents, but in 1891 
it was decreased to 78 cents per ton of ore crushed. 

The labor employed is thus distributed: The figures are ^ . 
per month of 30 working days, working 76 stamps ; one mill- ^^ 
man, $175; one assistant, $100 ; six feeders (three per shift) 
at $3 per day, $90 per month, $540 ; two concentrator men 
(one per shift) at $3 per day, $180; total, $995, or 38 cents 
per ton. The day of 24 hours is divided into two shifts. 

In looking down the columns of the comparative table it 
will be seen that while the mills are all of the same type, and 
are engaged in crushing ore of a generally similar character, ^^^C 
there are yet certain diflFerences which it will be interesting ( ' 
to inquire into. The stamps are all of comparatively light ^^V^ 
weight, varying from 500 to 600 pounds. This is rendered 
necessary by this particular style of milling where the high 
drop would be impracticable with stamps of 850 and 900 pounds, C . 
the average of other districts. The speed also is directly f^-^ 
affected by the same cause, for the work required to lift the 
stamps from 16 to 18 inches prevents the rate of drop exceed- 
ing 32 per minute for good work. A speed of 40 is probably 
the practical limit. 

The slowest drop is that of the New York mill, 26 per Cr 
minute, and the fastest is that of the Hidden Treasure, 32 per ^^^ 
minute. The latter represents the tendencies of the milling f&ra 
practice of to-day, while the former has in this respect 
adhered to older modes. It is to be noted in addition that the 
New York mill has the longest drop; in this respect, also, it 
follows the older practice, for the tendency to-day is certainly 
not in the direction of an increase of drop. The issue or 
depth of discharge ranges from a minimum of 11 inches at 
the Gregory Bobtail to a maximum of 16 at the Prize and 
Randolph mills. The 16-inch issue represents the older, 
deeper mortar, while that of the Gregory Bobtail fairly indi- 
cates the construction of the more recent plants. 

The crushing capacity of the mills varies from slightly over (^^ ^ 
to a little under one ton per stamp per 24 hours. Such varia- ^ ^/ 



^ 



c 



22 STAMP MILLING OF GOLD OBES. 

tion as exists is due to the differences in weight of stamp, 
height of drop, depth of discharge, speed of drop and fineness 
of screen. The Prize has the least crushing power per stamp, 
since it has a comparatively short drop, a deep discharge, and 
a weight of stamp which is slightly under the average. The 
deep discharge affects the crushing capacity of the Randolph. 
The slow drop of the New York is largely made up by the 
heavier stamps and the increased length of the drop. The 
different features mentioned are well balanced in the case of 
the Hidden Treasure, and it has, therefore, a crushing power 
slightly above the average. 

The percentage of concentrates obtained ranges from 12 to 
^^VCi y"^ i/ ^^ ^^^ cent., and forcibly indicates the refractory character of 
WujL^the millstuff. The value per ton of these concentrates is very 
low, averaging $10 to $12 net, and they are only rendered 
a profitable item by successful and cheap concentration supple- 
mented by light charges at the smelter. Neither the concen- 
trates nor the blanketings undergo further treatment at the 
mills. They are shipped on railway cars which are conveni- 
ently switched at the mill door, and are sent to the various 
smelting establishments at Denver. The freight is now $1.50 
per ton. Ninety -five per cent, of the silver and gold contents, 
as determined by assay, is paid for and $8 per ton is deducted 
as smelting charges.* At one time, when this class of 
material was less plentiful than now, a minimum rate of $7.25 
was allowed. 

The retort percentage is seen to average from 33 to 40 ; it 
depends upon the richness of the ore, since the gold in rich 
ore is coarser than that of the poor stuff, and hence is less 
perfectly amalgamated.! It is comparatively high for an ore 
the character of which is more fairly indicated by the fineness 
/ of the gold, which is comparatively low grade, varying from 

' '^^^Uaj 700 to 850 per thousand. The amount of water used in the mill 

varies from 1^ to 2^ gallons per minute per stamp, and is 
comparatively low owing to the small capacity of the mills 

 The charge varies according to the demand for such material. It has dimin- 
ished to $4.50 per ton since the above was written. 

•{■In squeezing the amalgam the millman can cause a variation of 10 per cent, 
in the quantity of gold obtained in retorting, according to the thoroughness 
with which he squeezes through the canvas the excess of mercury in the 
amalgam. 




'-^^It]^ 



GILPIN COUNTY, COLORADO. 23 

and the slight use made of blankets, together with the very high 
slope of the amalgamating tables, viz, from 1^ to 2^ inches per 
foot. 

In the matter of the wear of the screens and the loss of 
quicksilver, the divergencies are so considerable as to merit 
careful examination. 

The screens used in this district are of local manufacture and 
are made of planished iron,* size No. 24. The openings are 
straight, horizontal slots arranged alternately. The screens 
are graded, according to the size of the openings, from 1, the 
finest, to li, 2, 2^, etc. Numbers 1, 1^, and 2 are those gener- 
ally in use, and they are conceded to be equal to 60, 50, and 
40-wire mesh, respectively. It is to be noted, however, that 
this type of screen has nothing like the discharge surface of 
the supposed equivalent in wire mesh. This is seen in prac- 
tice where, in this district, for instance, a large proportion of 
the pulp is kept inside the battery until crushed to a size 
which would allow it to pass a 100-mesh wire screen. While 
these screens are by no means to be recommended for other 
districts, they are well adapted to the carrying out of the 
main idea underlying Gilpin County milling, namely, the 
retention of the pulp inside the mortar box for a long interval 
so as to allow of its amalgamation there. 

The side which carries the burr edge of the punched openings 
of the screen is always placed facing the inside of the mortar 
box, for the evident reason that the burr helps to break the 
pulp and so prevents the choking of the slots. It is the 
general custom of the mills to invert the screen after a time, 
since the passage of the pulp wears the lower portion faster 
than the upper. The screen is never reversed, that is, the 
original burr side placed facing outward. 

The life of the screen, the time it is used, in the different 
mills is seen to vary from a minimum of 16 days to a maxi- 
mum of 81, from two weeks to about three months. This very 
wide difference is far greater than can be explained by the 
greater or less attention of the millman and the extent to which 
he is willing to allow the screen openings to be enlarged by 
wear. The mills whose names appear in the comparative 
table are all situated at Black Hawk, and are all erected by the 

♦An American imitation of Russia iron. 




24 STAMP HILLING OF GOLD ORES. 

side of the creek which flows down the canon and under the 
muddy streets of the dirty old town. Put them in their order 
of succession, commencing with the Hidden Treasure, which is 
farthest up the creek, and an explanation of the widely differ- 
ing wear of the screens is at once suggested. 

MUL liife of screens— days. 

Hidden Treasure 81 

Prize 75 

Gregory Bobtail 00 

New York 25 

Randolph 16 

The Hidden Treasure receives water which is comparatively 
clean, and after having used it returns it to the creek, together 
with the addition of a certain percentage of sulphuric acid^ as 
sulphate of iron, derived from the contact of the water with 
the partially oxidized pyrites in the ore under conditions 
favorable to a certain amount of solution. The wash of the water 
in the battery, and the elevation of its temperature due to its 
rapid agitation under the stamps, are causes operating to aid 
a chemical action upon the metallic sulphides and sulphates 
in the ore. The water now reaches the Prize mill, where its 
slightly increased acidity reduces the life of the screens from 
81 days to 75. The Prize in turn contributes its share of 
sulphates, which again help to injure the screens of the 
Gregory Bobtail, which mill is a little farther down the 
creek. 

At this point the stream receives the very acid waters which 
issue from the underground workings of the Gregory mine, 
and in addition, before it reaches the New York mill, the 
water washes past banks whose sand is all more or less 
charged with partially oxidized pyrites, so that when it at 
length reaches the two lower mills the wear of the screens is 
inaasured by days instead of weeks. 

The acidulated battery water eats away the edges of the 
openings of the screens, and thereby rapidly decreases their 
time of service. 

Taking the case of the Hidden Treasure as the most typical, 
we find 432 tons of the ore are passed through a screen before 
it is considered worn out. This was the average for 1891. At 
Grass Valley a screen will live to pass through 200 tons, and 
at Bendigo (Australia), 134 tons. Among the factors tend- 




GILPIN COUNTY, COLORADO. 25 

ing to allow the screen a long life must be noted the very 
roomy character of the mortar boxes in this district, which 
diminishes the violence of the splash of the pulp. 

Coming to the question of the loss of mercury, we find it to qT 
vary from 3.7 to 9.8 dwts. per ton of ore crushed. The two mills ^-. ^ 
which show the greatest consumption of quicksilver have also ^^^^u 
the deepest issue, and it may be that the further flouring of the ^^^^ 
mercury added to the ore in the mortar box may be among J^ 

the causes of increased loss. The increased agitation under 
the stamp causes more " flouring," that is, the subdivision of ^ 
the mercury into minute globules, which become coated with ^t 
a film of foreign matter — the finely pulverized pyrites, for 
instance — which prevents their coalescing afterward, and so (j 

renders them readily borne away by the water to become a 
part of the value of the tailings-heaps in the creek outside 
the mill. This is probably so to a certain extent, but the vari- 
able loss of mercury at any given mill, and therefore probably 
between any two given mills, lies mostly in the fact that the 
more lots of ore that are treated the greater the expense under 
this head. All these mills are to a greater or less extent custom 
mills and treat ore coming from a large number of mines. 
Every lot requires separate crushing; when finished, the 
plates must be cleaned up each time, the amalgam collected 
and then retorted, operations all of which mean manipulation 
and consequent mechanical loss of mercury. The two mills 
showing the largest figures under this head of mill expendi- 
ture are the two which treat, more so than the others, a large 
number of small lots of ore. 

Before venturing upon a criticism of the methods of milling, 
it will be well to glance at the character of the ore. It has 
been stated that it is of the refractory class ; the percentage 
of concentrates obtained at the mills and the low grade of the 
bullion will have confirmed this statement. To give a more 
detailed description I will not scatter my observations over 
the districts, but endeavor to describe the ore of the Cali- 
fornia mine, treated at the Hidden Treasure mill, an ore 
which is of a representative character and is the product of a 
representative mine. 

The ore as sent down to the mill consists, roughly speaking, 
of 10 to 20 per cent, metallic sulphides, 15 to 20 per cent. 




26 STAMP MILLING OF GOLD ORES. 

quartz, 60 to 70 per cent, vein filling other than quartz. The^ 
lode matter consists of an altered form of the rocks enclosing 
the vein; the latter traverses for the most part a dike of 
andesite which is 17 feet thick. Occasionally the ordinary 
country rock, a granitoid gneiss, forms one or other of the 
walls of the lode and contributes to the vein filling. The 
filling of the vein consists, therefore, largely of feldspathic 
material. Of the metallic constituents, the sulphides, iron 
and copper pyrites, predominate. Gray copper (fahlerz or 
tetrahedrite) and galena are also present in noteworthy 
proportions. Blende and mispickel (arsenical pyrite) are 
sometimes seen, and chalybite (carbonate of iron) appears 
occasionally in the ore from the upper levels. 

The gray copper, which here is antimonial, is generally re- 
markably favorable to the presence of gold. This fact would 
prove a valuable index in selecting the ore were it not so 
often confounded with arsenical pyrite. Quartz, especially 
favorable when of a blue tint, is always associated with the 
pyrites in rich ore. 

The following results of assays made by me* upon a typical 

piece of ore broken in the 1,700 feet level will throw some 
light upon the distribution of the gold and silver in the ore. 



MINBKAL. GOLD. SIIiVEB. REMARKS. 

Oz. per ton. Oz. per ton. 

Iron pyrites .66 4.85 White, coarsely crystalline. 

Copper pyrites .85 53.50 Flaky, dark yellow. 

Gray copper .90 88.65 Chiefly covering the last. 

Blende .16 6.45 Black; crystalline. 

White quartz 3.32 7.35 | Opaque, massive, with small crys> 

Bluish quartz 8.56 5.84 ) tals of pyrites throughout. 

Flinty quartz .18 1.90 Brown, vitreous. 

Feldspathic gangue ... .90 2.35 Soft, granular, white. 



This analysis bears out the experience of the mills where 
half the gold contents of the ore are extracted by the first 
amalgamation in the mortar box. The gold can not, therefore^ 
be chemically combined with the pyrites ; on the other hand, 
the more highly mineralized the ore is the richer also it 
usually is. There is no doubt that the silver contents are for 
the most part associated with the copper-bearing minerals, 
while the gold is enclosed by the quartz, especially that 



*In 1886, when in the management of this mine. 



GILPIN COUNTY, COLORADO. 27 

quartz which is in immediate association with pyrites. 
Neither blende nor galena is an attendant upon the gold, and 
both are a nuisance in the mill. 

The following figures further illustrate the character of the 
ore; they represent the output for 1890: 150.44 tons of 
smelting ore, averaging $92.79 per ton net; 1,376.03 tons of 
concentrates, averaging $15.06 per ton net; 10,320.57 tons of 
millstuff, averaging $7.42 per ton. 

The " smelting ore" simply represents the heavily pyritic ore 
picked out at the mine and shipped direct to Denver. The 
millstuff yielded 4,766.39 ounces of bullion, worth $16.65 per 
ounce. Of the total tonnage 98 per cent, was mill ore, and 
of the total value 84 per cent. Of concentrates the mill ore 
yielded 13 per cent. 

Tests have been made from time to time to determine the 
completeness of the extraction at the mill. Herewith is 
given the result of one made by me in March, 1891 : 

A lot of 8,400 pounds was passed through the breaker and 
rolls erected at the mine by the company for the particular 
purpose of accurate sampling. These 8,400 pounds contained 
four per cent, moisture, leaving 8.064 pounds net. 

The assay gave : Gold, 1.85 ounces ; silver, 8.75 ounces per 
ton. 

The contents of the 8,064 pounds were therefore 7.96 ounces 
of gold and 35.28 ounces of silver. At the smelter such ore 
would be worth* net $26.97 per ton, and have a total value of 
$108.24. 

The ore was sent down to the mill and yielded, after treat- 
ment for 18 hours by five stamps, 6.70 ounces of bullion, 
worth $16 per ounce, or $107.20, and 2,325 pound's of concen- 
trates. The latter contained 15 per cent, of moisture, leav- 
ing 1,977 pounds net.f The assay of these concentrates gave 
1.76 ounces of gold and 10.34 ounces of silver per ton, or a 
total for the 1,977 pounds of 1.74 ounces gold and 10.22 ounces 
silver, worth $45.02, or deducting the freight to Denver (at 
$1.50 per ton) and smelter charges (at $8 per ton) there remains 
$35.63. 

* Smelting charges, ^$12 per ton and a return of tf6 per cent, of the gold and silver, 
according to the New York quotations. Silver is figured at %l per ouncQ and gold 
at 120. li'reight, $4.50 per ton. 

+ The ore therefore contained 24)^ per cent, of sulphurets. 



28 STAMP MILLING OF GOLD ORES. 

Let US compare the results. The milling was at the rate of 
84 cents per ton. Therefore the mill return was, after all 

deductions are made, $139.45. The bullion was $107.20, the 
concentrates $35.63 ; total, $142.83, from which deducting the 
milling cost at 84 cents per ton, $3.38, leaves $139.45. At the 
smelter the amount received, owing to the larger deductions 
and charges, reaches the smaller sum of $108.24. Commer- 
cially the mill therefore gave the mine-owners better returns 
than the smelter, although there is no doubt but that the 
latter extracted a larger percentage of the values in the ore. 

As a test of the millwork the figures work out as follows : 
There were in the ore 7.46 ounces of gold and 35.28 ounces of 
silver. There were extracted as bullion 5.25 ounces Au, 14 
ounces Ag; and in the concentrates 1.74 ounces Au, 10.22 
ounces Ag, or a total of 6.99 ounces Au, 24.22 ounces Ag. 
Thus the mill, including the value in the concentrates, saved 
93.8 per cent, of the gold and 71 per cent, of the silver. The 
mill did not, however, complete the extraction of the gold 
and silver in the concentrates, so that it actually obtained by 
amalgamation alone 70.4 per cent, of the gold and 39.7 per 
cent, of the silver. 

This test is very fairly representative of the returns obtained 
on a larger scale. Generally speaking, it has been found that 
the mill yields as many ounces of base bullion as there are 
ounces of pure gold in the ore as found by fire assay. The 
mill gold is 780 fine, and this proportion very nearly represents 
the percentage extracted by amalgamation. Considering the 
fact that the Gilpin County ore is probably the most highly 
charged with sulphurets of any of the gold-bearing millstuff 
treated by amalgamation at the chief mining centers of the 
present day, it is not too much to say that the extraction at 
the stamp mills is exceedingly good. 

That this is so is due to the proper recognition of the 
necessity for altering modes of treatment in accordance with 
differences in the character of the ore treated — the first 
principle of all successful milling. The Georgian and Cali- 
fornian types of stamp batteries were found unsuited so soon 
as ever the surface quartz had been pierced and the unoxidized 
pyritic ore was penetrated. The change from the fast-drop, 
shallow-discharge system to that which has been described 



A 



GILPIN COUNTY, COLORADO. 2ft 

did not take place in a day, but was the outcome of much 
hard, persevering experimental work. The result is seen in , 
the roomy mortars, slow drop and deep discharge which 
characterize the Black Hawk mills. 

These features are those best adapted to the extraction by "^ ?f^ 
amalgamation of the gold in the ores of this particular dis- '^ • 
trict.* The ore has been described, and it may be added that '*^«.va 
in this locality ordinary panning, such as that employed by 
every prospector, will give no *' colors" even with material 
which in the mill will yield rich returns, f The gold is in a 
very finely divided condition and very intimately associated 
with the pyrites. To bring about a separation fine crushing 
is a necessity, and to cause the combination of the gold so 
separated with the mercury, time must be allowed for it to 
sink to the bottom of the mortar box where the mercury 
chiefly lies. 

The slow drop and deep discharge produce a pulverization 
which is even finer than is indicated by the size of the screen 
openings, lor it is found that about 70 per cent, of the pulp 
will pass through a 100-mesh wire sieve.J Of the pulp a 
preponderating part is pyrite, which is thus crushed very 
finely because the deep discharge and roomy character of the 
mortar allow it to remain inside after it has been pulverized 
to a size which would allow of its passage through the screen. 
This fact is of great assistance in causing the fine gold to be 
separated from the pyrite, with which it is chiefly associated. 

It may appear an error to allow the pulp to remain inside 
the battery after it has been pulverized sufficiently to allow 
of its expulsion through the screen. If pulverization were 
all that is aimed at this would indeed be a mistake, but the 
battery is here an amalgamating as well as a crushing machine. 
The delay which occurs before the pulp is expelled enables 
the gold which has been separated from the pyrite to become 
amalgamated by contact first with the free mercury intro- 
duced by the feeder, and secondly by being splashed against 

* See also Chapter I. If stamp miUing Is to be used this is the best way of 
using it on these ores; whether another method, such as concentration, would 
be better, is quite another question. 

•fOf course, In this statement I exclude the oxidized quartz ore of the surface 
portions of the veins. 

X This fact has already been explained as being in part due to the particular 
variety of screen employed. 



30 STAMP MILUNO OF GOLD OSES. 

the plates at the front and back of the mortar. The roomy 
character of the mortar is here necessary, for in a mortar 
whose construction allowed of less space between the dies and 
the 8creen, or between the dies and the back of the mortar, 
the violent splash of the pulp would cause a scouring of the 
inside plates, which would rapidly remove the coating of 
amalgam and render the plates useless as gold savers. Thus, 
to summarize, the deep discharge causes a fine pulverization 
of the ore, and the long interval between the drops, which 
with a speed of 30 per minute is noteworthy, allows of the 



ailpin County Bumping Table. 

action of gravity among the particles of the pulp. The deep 
discharge and roomy mortar aid in preventing the production 
of a too violent splash, and in permitting the pulp to remain 
inside the battery until the amalgamation required has been 
effected. Thus all three features — roomy mortar, long drop, 
and deep discharge — are seen to supplement one another until 
the proper conditions are obtained. 

Black Hawk is the most important gold-milling center in 
Colorado, and the group of machines whose principle features 
we have considered is in eveiy way representative of the 
milling practice. Among the factors tending to increase the 
coat of handling the ore must be mentioned the universally 
bad situation of the stamp mills. They are placed in the flat 



GILPIN COUNTY, COLORADO. 31 

•of the gulch away from the hillside, the consequent want of 
fall rendering impossible the erection of suitable ore-bins and 
feeding machinery. This is a fault which may be partially 
condoned on the ground of their erection having been by no 
means recent, and is largely due to the desire to utilize as 
much as possible the motive power in the creek. 

The mills are out of date in being unsupplied with rock- 
breakers. It is true that the advantage derived from the use 
of a rock-breaker is largely discounted by the fact that the 
position of the mill buildings would prevent its being supple- 
mented by grizzlies (or sizing bars) and ore bins. At the 
Hidden Treasure there were a few years ago three small rock- 
breakers, but owing to the consumption of motive power and 
the awkwardness* of feeding them they have given way to a 
return of the more primitive methods of the sledge hammer. 

The mills, as we have seen, crush very slowly, and hence 
there is not that crying need for a rock-breaker which exists 
in a Californian or Australian mill, but nevertheless there is no 
doubt that in this respect the Gilpin County batteries are 
defective, for apart from the improvement in the feeding which 
follows the introduction of a breaker, the irregular breaking 
of the ore by the sledge hammer must tend largely to increase 
the strain upon the mill machinery. This is rendered very 
evident by noting the wear and tear of the shoes and dies, 
which in the mills of this section are excessive. 

In the matter of feeding it is safe to say that given hand- 
feeding which is conscientious, it is superior to that of a ^'-^i.J 
machine. But man is human, and an occasional pipe or a 
casual nap are both temptations known to feeders as to other 
men. On the score of regular and accurate feeding the 
automatic machine is, therefore, preferable to the average 
man. 

Where stamps crush fast the self-feeder is a great economy. 
How does this side of the question appear in Gilpin County ? 
We find that the low-crushing capacity of the mills enables 
one man to keep 25 heads supplied. On the score of economy 
one would therefore, at first sight, acquit the mills of this dis- 
trict ; but if the figures are examined a different tale is told. 

*Owing to the construotlon of the mill the ore had to be shoveled up into the 
breakers. 



Jl 



32 STAMP MILLING OF GOLD ORES. 

For 75 stamps the cost of feeding comes to a total of $6,500 
per year, while on the other hand, if the mill were supplied 
with the most expensive type of self-feeder, the cost of the 
additional plant would not be over $4,000. On the score of 
eflSciency I can vouch for the fact that the feeding is regularly 
and intelligently done, and, with well-tried workmen, leaves 
but little to be desired. Feeding machines are of but little use 
unless preceded by grizzlies and rock-breakers ; and therefore,, 
notwithstanding a natural aversion to methods which are out 
of date and machinery which is incomplete, and recognizing the 
unfortunate position of the mill buildings, a position chosen 
in the days preceding the introduction of improved labor- 
saving appliances, I can not say that, either from a share- 
holder's or millman's point of view, the arrangement of the 
mills can be advantageously altered. When new mills are 
built it is to be hoped, now that the mule-wagon as a trans- 
porting agency has begun to give way to a steam tramway,, 
that they will be erected in well-chosen sites, giving the fall 
requisite for the labor-saving machines which have been tried 
and not found wanting since the days when the Black Hawk 
batteries were first put up. * 

The Black Hawk millman is as interesting and important a 
feature of the milling as the machine whose working he 
directs. He has been trained in the best of schools, that of 
experience, and this factor in making him a good workman 
has been supplemented by the necessities of a continual 
watch over the treatment of an ore which is subject to a great 
variety of mineralogical constitution. Two other points are 
worthy of notice in this connection. All the mills ara to a 
more or less extent dependent for their full supply of ore 
upon mines other than those of the owners of the mill itself. 
The custom milling which characterizes Black Hawk is a 
power in making both proprietor and millman careful in the 
treatment of the ore and wide-awake for possible improve- 
ments in the methods employed. Further, the milling is 
recognized to be as important as the mining. This may seem 
an unnecessary statement, but 1 have known instances where 

♦The stamp mills recently designed in Denver or Central City are equipped 
with rock-breakers, ore-feeders, etc., and leave no room for such criticism as is 
invited by the old Black Hawk plants. 



GILPIN COUNTY, COLORADO. 33 

a good mine well managed has owned a mill whose working 
has been under the direction of a man who may have been a 
good miner, a good chemist, anything you will, but who most 
assuredly was a bad and inexperienced millman. In Australia 
I have seen a first-class 40- stamp battery consigned to the 
tender mercies of an engine driver, who, in addition to attend- 
ing to the machine which gave the power to work the stamps, 
was supposed to have the general millwork under his charge. 

In Gilpin County the work of management is not con- 
sidered to finish at the shaft mouth ; on the contrary, the 
milling demands the greater attention. The millmen are 
better paid than the mine foremen. The needs of the district 
have produced men who are more fully conversant with the 
bed-rock principles of gold milling than those of almost any 
other mining center, and such men are not too well paid. 
Custom milling has had its eflFect in making proprietors 
anxious, by placing good men in charge, to gain the confidence 
of the mining community, and has reacted upon the millmen 
themselves by encouraging competition in doing good work. 

The foregoing description of the methods of this district 
and the commendation of the skill and intelligence of the 
millmen must not be construed as an indiscriminate approval 
of the practice in vogue at Black Hawk. If the stamp mill is 
to be used, then the way it is used in this locality, for the par- 
ticular ores of the locality, is correct, but this does not pre- 
vent the discussion of the question whether concentration, 
with or without amalgamation, may not be better than stamp 
milling. As a matter of fact, the ores of the district are of a 
noteworthy variety, and in occasional cases the shipment of 
them direct to the smelter would give better commercial 
returns; in other instances concentration followed by the 
stamping and amalgamation of the jig tailings would doubtless 
prove advantageous, and, indeed, in rare cases straight concen- 
tration might suffice. 

This opens up a wide subject, and one which can not be con- 
sidered on the present occasion. The writer simply desires to 
emphasize the fact that for a large proportion of the output 
of Gilpin County the existing practice of stamping plus 
amalgamation plus concentration is the one best adapted to 



34 STAMP MILLING OF GOLD ORES. 

the peculiar local conditions, and that however incomplete and 
badly arranged the mechanism of the mills may be, the idea 
of the amalgamation of the gold in these highly pyritic ores 
by means of slow-dropping stamps, working in deep roomy 
mortars, is one warranted by theory and corroborated by 
experience. 



CHAPTER III. 
Thb Typical Stamp Mills of California. 

The history of Grass Valley forms one of the most impor- 
tant chapters in the record of gold mining in America. Grass 
Valley is the mining center of Nevada County, and that 
county is the leading gold-producing region of California, 
covering about 20 miles of the length of the main gold belt. 
To the north it is bounded by the South Yuba and the Bear 
Kiver, and to the south by the Middle Yuba, names which 
have become classic in the story of gold discovery. 

The pretty town of Grass Valley has a population of 6,500, 
mostly Cornishmen. It lies among the foothills of the Sierras, 
and is about 150 miles northeast of San Francisco. The 
earliest settlement took place in the fall of 1849. The placers 
which were at that time discovered proved to be of great 
richness. In June, 1850, the .first quartz ledge was found, but 
its value was not realized at that time. In the following 
October, however, croppings of extraordinary richness were 
discovered on Gold Hill, and created a sensation which led to 
the commencement of vigorous prospecting. Several lodes 
were then uncovered on Massachusetts Hill, Eureka Mountain, 
Ophir Hill, and other localities which have since proved very 
productive. 

The first mill built in this district was erected in January, 
1851, on the west bank of Wolf Creek, nearly opposite the 
site of the present Empire mill. This is said to have been the 
second stamp mill erected in the State of California, priority 
being conceded to a plant built in Mariposa County in 1850. 
The latter consisted of eight round stamps driven by water- 
power. Each stamp occupied a single mortar. Mr. Melville 
Attwood informed the writer that these stamps revolved, and 
were the originals upon which the typical Oalifornian stamp 
was subsequently modeled. 

(85) 



36 



STAMP MILLING OF GOLD ORES. 



From 1861 to 1866 the Grass Valley district is estimated to 
have yielded gold of a value to exceed $23,000,000. The sum 
total to date has been calculated to be no less than $100,- 
000,000. The eleventh census (1890) gave the output for that 
year as $1,715,248, derived from quartz lodes, and $203,331 
obtained from placers, the total being thus nearly $2,000,000. 
At that time 58 mines were active and 295 stamps were at 
work. The mint report gives the yield for 1892 as $1,945,406 
against a reported production for the preceding year of 
$2,207,887. It is estimated, however, that the output for 1892 
was as much as, if not more than, that of 1891, so that we 
may put down the production at about $2,250,000. At the 
present time the mining industry of this old district is in a 
healthy and vigorous condition. In 1895 the production of 
Nevada County was $1,789,815. 



COMPARATIVB TABLE. 





00 


of each 

• 


of drops 
nute. 


• 

P. 

2 
%* 



.d 


• 

© 




tyof 
mill. 


Name of mill. 


bi a 

© J7 


■as 


^a 


o9 


5o& 


^§2 






^1 


as 

d o« 


be 
© 


©"O 


rusl 
cap 
sta 


rus] 
cup 
ent 




^ 


:z5 


» 


p 


O 


O 




• 


Lbs. 




In. 


In. 


Tons. 


Tons. 


North Star 


40 
40 


850 
850 


84 
03 


7 
7 


4 

4^ 


1.6 
1.5 


04 


Empire .... 

Idaho 


(JO 


40 
10 


850 
750 


05 

90 


7 
6 


4 
6 


2.0 
1.7 


80 


W. Y. O. D 


17 



Description of screen. 

f 


Fineness of 
screen. 


Percentage of 
concentrates. 


Value of concen- 
trates per ton. 


Rotort percent- 
age. 


Fineness of 
bullion. 


Consumption of 
mercury per 
ton of ore. 


• 
OQ 

d 
© 

o 
© 


Consumption of 
water per 
stamp per min. 




Mesh. 


Per 

cent. 


Dol- 
lars. 


Per 
cent. 


Per 

1,000 


Dwts. 


Days. 


Oals. 


North Star— Perf. tin plate . . 
Empire— Perf. tin plate .... 
Idaho— Brass wire cloth . . . 
W. Y. O. D.— Perf. tin plate . . 


80 
90 
40 
40 


8 

fi 

2H 


60 

80 

85 

100 


40 
40 
42 

88 


851 
820 
840 
850 


14^ 

ll ' 
11 


24 
15 
11 
14 


4 



The foregoing comparative table gives the figures which 
best indicate the main characteristics of the milling practice. 

Of the four plants whose names appear on the list, the North 
Star is the one which will be taken as a type of the stamp 



TYPICAL STAMP MILLS OP CALIFORNIA. 37 

mills of Grass Valley. The North Star mine was first worked 
in 1850. In the following year a party of Frenchmen organ- 
ized a company known by the name of the "Helvetia & 
Lafayette." In 1857 the mine obtained the name which it 
now bears. A 16-stamp mill was erected in 1866. The mine 
has been closed down and reopened at various intervals. 
Since 1884, however, work has been continuous. The present 
mill, which was erected in 1886, contains 40 stamps, and its 
parts are so arranged as to give to a maximum degree an 
automatic handling of the ore. The mill is often referred to 
as typical of the best results of Oalifornian experience, and 
it deserves its high reputation. 

A clear "*dea of the arrangement of the parts of the plant 
will be best obtained by following the ore in its passage from 
the entrance at the top of the mill building to its exit as 
waste at the bottom. The ore arrives from the mine shaft in 
a car, holding about two-thirds of a ton of ore, and is emptied 
upon grizzlies or sizing-bars which separate the fine stuff 
from the large lumps. The former falls through the inter- 
spaces, and goes into the lower ore-bins for fine ore, while the 
latter passes into the upper ore-bins above the rock-breakers. 
There are eight sets of grizzlies, inclined at an angle of 40 
degrees, each set consisting of 15 bars of iron, 12 feet long, 
i inch wide and separated by spaces of 2^ inches. Recently 
the spaces have been diminished to 2 inches, thereby increas- 
ing the fineness of the ore supplied to the stamps. 

The upper ore-bins are three in number and feed three rock- . v 
breakers arranged in a row, one beneath each ore-bin. The / j /; 
breakers are all of the Blake pattern, having jaws 15 by 9 
inches, and they are fed by means of an ordinary iron shoot. 
The feeding is regulated by the millman who adjusts the 
gate opening. 

The millstuff as it is reduced by the rock-breaker falls into - 
the lower or fine ore-bins which supply the stamps. The ore ^'^'^^^ 
is fed to the stamps by means of Hendy Challenge feeders, of 
which machines there are eight, one to each 5-stamp battery. 

Each stamp weighs about 850 pounds. The total weight is ^ U^, 
thus distributed: Stem, 358; head, 228; tappet, 112; shoe, 
152 pounds. The stamps drop from 82 to 85 times per minute 
through a height of from 6 to 8 inches. Each mortar con- ^^^s-fi 



U^ 



38 STAMP MILLING OF GOLD ORES. 

tains 5 stamps, and each such group (called a battery of five 
heads) crushes about 8 tons per 24 hours, being at the rate of 
1.6 tons per stamp. 
^Utl The depth of discharge, the distance from the bottom of the 

^ . t screen to the top of the die, varies from a minimum of 2 

tK; inches to a maximum of 6 inches. No serious effort is made 
to maintain anything like a uniform issue. The crushed ore 
passes through screens made of tin plate and perforated with 
holes of such a size and number as make them, it is supposed, 
equivalent to a 30-mesh wire cloth. 
r, f\ /> The pulp is discharged upon amalgamating tables, which 

^ are subdivided into three consecutive divisions, termed, 

respectively, the battery, apron, and sluice-plates. They are 
all covered with sheets of copper i inch thick, electroplated 
with silver at the rate of 1 ounce of silver per square foot of 
copper. 
(^^ ^ ^ From the amalgamating tables the pulp passes to the con- 
^^^^^^^^^iv4> centrators upon the floor below. The discharge from two 
batteries passes direct to the concentrators, but that from the 
other six flows first over Rittinger shaking tables, intended to 
catch any escaping amalgam. The concentration plant con- 
sists of 4 Frue vanners and 12 Triumphs, being in the usual 
proportion of two concentrators to each battery. The con- 
centrators are run at a speed which gives them from 200 to 
230 strokes per minute. 

The entire machinery of the mill is propelled by water- 
power. Ninety-three miner's inches (one inch being equal 
to 1.574 cubic feet, or 11.77 gallons per minute) under a head 
of 277 feet and a pressure of 212 to 215 pounds per square 
inch, serve to work a 6-feet Pelton wheel which drives the 
stamps. Twenty inches of water propelling a 4-feet Pelton 
run the rock-breakers ; and 12 inches, with a 3-f eet Pelton, 
work the concentrators. The transmission of power from the 
waterwheels is effected by manilla ropes, 1| inches in 
diameter. 
Such is the general arrangement of the plant. The foUow- 
^'l ing additional details will prove of interest : The stamps in 
^roJ^ each battery drop in the order of 1, 4, 2, 5, 3. In watching 
^ them I found that a stamp often makes a complete turn in 
^'•^-MM^ three drops; on the other hand, it occasionally falls several 



^^^^1>V '* 



. - , - il 



N 



1" 
t 



\ -'1, 

\ : : 



/ 



/' 



/ 

/ 



I, 



/.. 



// 






l> 

i 

1 

i; 



/ / 









J 



TYPICAL STAMP MILLS OF CALIFORNIA. 39 

times without making an appreciable turn. On an average it 
requires five drops to make a complete revolution. The tap- 7 
pets are keyed, not screwed, upon the stem. Screw tappets '/^<tj 
were used long ago, and discarded. 

The shoes and dies have the following dimensions : Shoe, 9 ry 
inches diameter, 8 inches high, with a tongue 3^ inches thick ; ^^^^a.^ 
die, 9 inches diameter, 5 inches high, with a seat \\ inches ^ y 

thick. The former is made of chrome steel, obtained from ji ' 
Brooklyn, N. Y. ; the latter is of cast iron from the local ^-Mj 
foundry. The remnants from wornout shoes are used in Orass 
Valley, being added to the iron of the dies, so that the latter 
contains about 20 per cent, chrome steel scrap. The steel 
costs 6 cents now, but used formerly to cost 9 cents, delivered. 
The cast iron is delivered at the mill for 4^ cents per pound. 
The remnants of both shoes and dies are sold to the local 
foundry for 1^ cents per pound. 

The average weight of the shoe is 152 pounds ; when worn 
out it averages 48 pounds. It gives a service of 143 days, and 
therefore wears at the rate of 7.3 ounces of steel per ton of 
ore crushed. The average weight of the die is 93 pounds, 
which is decreased to 45 pounds when it is worn out, that is, 
after 55 days of service.* It wears therefore at the rate of 
8.7 ounces of iron per ton of ore. Cast-iron dies, when used 
in conjunction with steel shoes, are found .to produce a more 
even wearing surface than when steel falls upon steel. The 
cupping or irregular wear just referred to diminishes the 
crushing capacity of the stamp. 

Dies of 5 and 4 inches depth have both been used. The 
use of the former as compared with the latter means an 
economy of the iron, since the portion finally discarded is 
similar in both cases ; but on the other hand, it causes a greater 
variation in the depth of discharge as the die wears down. 
This objection can, however, be overcome by employing some 
device, and there are many that will serve to maintain a con- 
stant height of issue. 

The mortars are of a pattern common in California. The ' . 
internal dimensions at the level of the discharge are as fol- ' . . , 
lows: Inside length, 4 feet 4i inches; inside width, 17^ 

*For these figures, as indeed for most of the details given, I am Indebted to the 
courtesy of the manager, Mr. Emile R. Abadie. 



40 STAMP MnxiKa of gold obes. 

inches ; from the screen to the die is 6 inches ; from the side 
of the mortar to the nearest die f inch; from the back ot 
the mortar to the die 2| inches, and between dies J inch. 
The depth of the mortar below the bottom of the screen is 7 
inches. The feedhole comes immediately behind the three 
middle stamps and gradually widens the upper part of the 
mortar. The latter is lined with steel plate 1 inch thick. 

There is one inside front amalgamating plate. Its use was 
introduced in 1888. It is of silver-plated copper, and is 52 
inches long by 4^ inches wide. It is screwed down to a chuck- 
block, a wedge of wood which fits tightly against the front of 
the mortar, and slopes toward the interior at an angle of 45 
degrees. See accompanying sketch, Fig. 1, where the distance 
A £ 18 4i inches, and A C6 inches. 
r\ The screen frame has four partitions which divide the dis- 

^^ ^lu , J ^ , charge into five parts. Each division is 9 inches wide and 12| 

inches high. Each partition is 1| inches broad. The frame 
itself is 4 feet 4 inches long by 18 inches wide. The parti- 
tions referred to (see Fig. 2) serve the purpose of strength- 
ening the screen, but they obstruct the discharge and cause a 
loss (including the ends) of about one square foot of surface. 
, From the bottom of the screen there is a drop of 6 inches 

, ^^ /^ f to the battery plate. The latter is 4 feet 2 inches wide, and 

^ extends for 18 inches. It is held in an iron frame (D E) 
which is bolted to the mortar. From the battery plate 
the pulp passes into a trough, then through a distributor 
consisting of a vertical iron partition pierced by 20 holes, 
each i inch. There then follows a drop of 3^ inches to the 
apron-plate. The apron is 4 feet 5 inches wide for a length of 
2 feet 6 inches, then it becomes beveled for the remaining 
length of 2 feet, and decreases to a width of 22 inches before 
' < WL. discharging on to the sluices. The apron has a slope of 1^ inches 
per foot. The sluices are 22 inches wide and 12 feet long, and 
have a slope of 1 inch per foot. They deliver the pulp to shak- 
^^ j.ng tables (lined with copper) of the Rittinger type. In the case 
of two batteries, however, these tables have been thrown out, 
the narrow sluice-plates have also been discarded, and instead 
the apron has been extended at almost its full width, making 
a total length of 16 feet, of which the first 2^ feet is 53 inches 
wide and the remainder 46 inches. The change was made 



S 






I ^ ^,^ 



TYPICAL STAMP MILLS OF CALIFORNIA. 



FlK-].— Mortar UBedal the North Star Mill, Oraaa Valley, CbI. 



4Z 8TAHP MILLING OF GOLD ORBS. 

after my first visit to the mill in 1886, when it appeared to 
me that the bad arrangement of the amalgamating tables waB 
a very great blemish of an otherwise splendid milling plant. 
The manager tells me that the alteration is a decided improve- 
ment. The accompanying reproductions from photographs* 
will illustrate the old and the new arrangement as run side 
by side. The sketch on opposite page, Fig. 3, shows the out- 
lines of the plates, contraating the old and new style by 
indicating the former in dotted lines. 



Needless to say, the change which has been made is a step 
in the right direction. Even now, however, the diminution 
in width from 53 to 46 inches appears to me to be a mistake. 
The width should increase rather than the contrary, for the 
same amount of pulp and water passes over the lower plates 
as over the upper ; and the gold to be caught by the lower 
end of the tables is more fine and more readily carried away 
than that which is caught at the top. I noticed that in the 
case of the narrow (22-inch) sluices the amalgamating sur- 

•Whlch I owe to the kindness of Professor Christy, of the UnlverBltyoI Call- 



TYPICAL STAMP HILLS OF CALIFORNIA. 



43 



face had been rubbed off (scoured) at the sides, exposing the 
naked copper ; thus proving the increased friction due to the 
more rapid flow of the pulp, caused by its confinement to so 
narrow a space. How does the millman expect to catch fine 
gold under such conditions? 



Mortar 




FDTstribufceto 



K- l^2^->t 



Slijiice 
PIkte 



— i-' 



46^-4—  






Fig. a 



*— .. 

AMALQAMATINQ TABLES 

ATTHE NORTH STAR MtLL 
(dO. S) 



The pulp from the amalgamating tables is distributed to 
the concentrators arranged on the lower floor. It is inter- 
esting to note that the experience at this mill is in accord 
with that of the other plants in this district, namely, that the 



^^•■fv 



J 

I 



A| 



'f 



vt 



' . f 



44 STAMP MILLING OP GOLD OBES. 

Frue vanner requires less attention and gives less trouble 
than the other endless belt concentrators. It is stated by the 
manager that at this mill, if all the machines were of the Frue 
pattern, the mill labor would be decreased by one man per day.. 
Mercury is added to the ore in the battery at the rate of 
from one-quarter to one full teaspoonful every hour. Upon 
y),|i^ ^ weighing several of these teaspoonfuls they were found to 

• V)jJ average 2 ounces troy. The amount to be added to the ore is 
judged by the appearance of the amalgamating tables. If the 
amalgam is very soft and the mercury tends to separate out 
in globules, then it is clear that the amalgam is more than 
saturated, and the millman diminishes his dose. If, on the 
contrary, the amalgam is dry — that is, becomes hard and 
lumpy — it is evident that more mercury is required. 

The proportion of amalgam obtained respectively inside and 
outside of the mortar varies from time to time, and is depend- 
'j ent upon the richness of the ore, its hardness and grain, the 
' fineness of the screen and the changing depth of discharge. 

The following figures will illustrate this fact. When there 
were obtained outside of the battery, on the amalgamating 
tables, 767, 764, 747, 261, 268, 618, 205, and 200 ounces of 
amalgam, there were cleaned up from the battery residues 
and inside plate, the following corresponding amounts : 628, 
1,811, 1,030, 459, 456, 1,640, 741, and 732 ounces. The mean 
ratio is, therefore, two to one in favor of the inside. 

The amalgam retorts about 40 per cent. That obtained 
from the tables has an average value of $6.27 per ounce, 
equivalent to a gold percentage of 35. That which comes 
from the inside of the battery is usually worth $8.20 per 
ounce, equivalent to 47 per cent. gold. Thus, for instance, 
from 764 ounces of amalgam obtained from the outside plates 
there were obtained 274 ounces of retorted gold, which on 
melting gave 269 ounces of clean bullion. Similarly 1,811 
ounces amalgam from the inside clean-up gave 795 ounces 
retorted, or 784 ounces melted, gold. The bullion is worth on 
an average $17.60 per ounce, equal to a fineness of 0.851. In 
1892 ten flasks of mercury were consumed in the treatment 
of 15,360 tons of ore ; this is at the rate of 14^ dwts. per ton. 
(j^ ^ n The general clean-up is bi-monthly. The inside plates are 

p not touched save at that time. The outside plates are scraped 



(,'uV 



TYPICAL STAMP MILLS OF CALIFOBNIA. 45 

and dressed every morning. The fortnightly clean-up is com- 
menced at 8 A.M., and is completed by 2 P.M. Each battery 
of five heads is stopped in turn. The dies are taken out, and 
the residues are fed into the one particular mortar which is 
the last to be cleaned up. The final accumulation from this 
battery is removed in buckets, and washed in an ordinary 
prospector's pan. The iron chips, from the abrasion of the 
shoes and dies, as well as from that of the drills which are 
used underground, are removed by a magnet. The headings 
obtained from the washing of the battery residues are then 
treated in a small grinding pan — ^28 inches diameter and 9 
inches deep— provided with a false bottom 2 inches thick. 
The rich residues are subjected to slow grinding, quicksilver 
being added to collect the gold as it is liberated. The result- 
ing amalgam is removed and washed in warm water. The 
tailings from the pan are introduced into a barrel whose out- 
side measurements are 2 feet diameter and 3 feet 9 inches 
length. No more mercury is added, there being already 
sufficient for the purpose in the pan tailings. Pieces of scrap 
iron, generally in the form of old nuts, bolts, etc., are added 
in order to serve as grinders. Cold water is used. 

The North Star mill is managed in a very economical and 
sensible way. Six men compose the total force employed in 
24 hours. The following details will be of service : 

1 head amalgamator, by day $125.00 per month, 12-hour shift. 

1 assistant " " night 100.00 " " " " 

1 rock-breaker man, " day 75.00 " " 10-hour " 

1 mechanic, " day 2.25 " shift " " 

2 concentrator men, day and night . . 8.00 *' " " " 

The last three on the list are included in the force attend- 
ing to the concentration. Their united wages average $220 
per month. 

Water-power costs at the rate of 18 cents per inch. The 
following figures, given to me by the superintendent, will 
indicate the distribution of cost : 

1888. 1889. 1890. 

Amount of ore crushed .... 17,269 tons 20,525 tons 14,414 torn 

Total costs 118,826 |16,798 |12,144 

Ck)St per ton 77 cents 81 cents 84 cents 

DETAILS OF GENERAL COST: 

Water 16,280.86 16,897.75 $4^48.00 

Supplies 2,740.48 4,829.47 2,875.81 

liEbor 6»866.40 6,066.17 6,425.80 



46 STAMP MILLING OF OOLD ORES. 

DETAILS OP supplies: 

Mercury I 508.50 $ 609.80 607.25 

Shoes and dies 1,188.40 2,502.28 800.61 

Other mill castings 260.79 479.97 09.74 

Screens 158.00 109.82 173.52 

Sundries 629.74 470.10 666.1ft 

DETAILS OF LABOR: 

Concentrators 12,527.25 12,560.50 2,377.00 

Amalgamators 2,112.00 2,569.17 2,238.80 

Rock-breaker 711.15 946.50 810,00 

Sundries 5.00 ... ... 

The mean average figures for this period of three years are 
as follows : The total cost per ton is 81 cents, made up of the 
three items : Water, 31 cents ; supplies, 18 cents ; and labor, 
32 cents. For mercury, the cost was 3.38 cents per ton; 
shoes and dies, 8.84 cents ; mill castings, 1.55 cents ; screens, 
0.95 cents; and sundries, 3.36 cents. In the matter of labor 
there is the following distribution : Rock-breaking, 4.73 cents ; 
amalgamating, 13.23 cents; and concentrating, 13.53 cents. 
It is interesting to note that the labor cost in the concentra- 
tion department exceeds that of amalgamation. While these 
figures do not by any means come up to some which are often 
quoted to show how cheaply gold ores can, under favorable 
circumstances, be worked, yet having due regard to the con- 
ditions obtaining at the North Star mine they are undoubtedly 
excellent. The ore and its encasing rock are of more than 
usual hardness, and this fact prevents that rapid crushing 
and minimum wear and tear which help so much to reduce 
the cost of treatment at other mills. 

The Empire mill is the older plant, having been erected in 
1883. It treats the ore of a mine which has been a very 
steady and large producer. There are 40 stamps arranged in 
eight batteries of five heads. Each stamp weighs 850 pounds 
and drops 90 to 95 times per minute through a height of 7 
inches. Each battery crushes 7^ tons per day, being at the 
rate of 1^ tons per stamp. The ore is very hard. Two rock- 
breakers and 8 Challenge ore-feeders are employed. A 5-foot 
Pel ton wheel supplies the motive power. It is under such 
control that the entire mill can be stopped in 15 to 20 seconds. 
k'i"^. The depth of discharge varies from 3^ to 5 inches. The 

tendency of late has been to diminish it because the mine has 
been unable to produce ore of such a high grade as formerly^ 
and economical reasons made fast crushing necessary. The 






TYPICAL STAMP MILLS OF CALIFORNIA. 47 

discharge has been gradually reduced from 6 inches to an 
average of 4^ inches. As the dies wear down old dies are 
placed underneath, so as to maintain the height of the issue 
within certain limits. 



North Star Mill, Orasa Valley, Cal. 

Formerly iron plates were used as false bottoms, but they 
were found to break too often. In order to further regulate 
the discharge, wooden cleats are fixed to the bottom of the 
front of the screen. As the dies wear down they are removed. 
The chuck-block or wooden wedge to which the single amal- 
gamating plate is attached has a straight face. Originally it 



' 48 BTAUP HUiLISGt OF GtOLD OSES. 

was convex, but this was found to bring the plate too near to 
tbe stamp and to injure its surface. The plate is 4 feet 2 
inches long by 4 inches wide. It is of plain copper. 

Of the total amalgam obtained in the mill an average of 75 
per cent, comes from the inside of the mortar. The variation 
is between 60 and 85 per cent. The tendency of late years 
has been to increase the proportion saved inside the battery 
by using finer screens. 



Nortb Star Mill, Qraas Valley, Cal. 

. Until lately brass wire-cloth screens, 30 and 40 mesh, were 

>^ (Ajjjai J "Sfid, but they have been replaced by perforated tin plate of 
an equivalent fineness. The surface of the tin plate is 
smoked on the blacksmith's forge, the idea being that this 
will prevent the adhesion of amalgam. The tin wears oif in 
about a week. The brass wire is said to cost at the rate of 




TYPICAL STAMP MILLS OF CALIFORNIA. 49 

$1.65 per screen and to give a service of 25 working days, 
while the tin-plate screens cost 55 cents each and last for 15 
days. 

From the mortar the pulp passes out upon the amalgamat- 
ing tables. The battery plate is 4 feet 2 inches wide and 2 
feet long. It has a slope of 2| inches per foot. The apron, 
which follows, has a width equal to that of the battery plate, 
but it narrows to 4 feet and then is beveled so as to diminish 
to 2 feet. The gradient is 1| inches per foot. The sluices are 2 
feet wide and 12 feet long. They slope at the rate of 1 inch 
per foot. All these amalgamating tables are covered with 
silver-plated copper having 2| ounces of silver per square foot 
of copper. 

The pulp then goes to the concentrators — 16 Triumph (^ 
machines. It is said that they give more trouble than the ^^^^^^z^" 
Frues, but it must be added that at this mill the introduction *^* 

of the Triumph concentrator was due to the result of a test 
extending over 60 days made between the Frue and the 
Triumph. As is often the case, however, in these matters, 
the trial was not altogether conclusive, because it is claimed 
that the result was largely due to the superior skill of the 
man who had charge of the working of the Triumph. 

The concentrates have a gross value of from $75 to $300 per ^ 
ton. The average is from $75 to $100. The yield of sul- ^^^^^n 
phurets is at the rate of 2 to 2i per cent. The clean-up is r 

made bimonthly. It is begun at 6.30 A.M., and the amalgam 
has been retorted and the gold melted by 4.30 P.M. Any 
replacing of new for old shoes and dies, if required, is done at ^-^ 
this time. This mill employs chrome steel for both shoes and ^</ ^ , 
dies. When the supply of steel runs short, cast iron from the /^^ ^*^ 
local foundry is used in its place. The service of the chrome ^^ 

steel is very good, giving a regular and even wearing surface. 
There is no marked difference to be noted in this respect 
between the chrome steel and the local castings, the former 
being more expensive, but wearing longer than the latter. 
The millmen prefer the steel because its longer time of ser- 
vice necessitates less frequent changes to be made, and there- 
fore gives them less labor. -{ ( , 

The mill, including the concentrators, uses from 12 to 15 ^L^ 

miner's inches of water. This is equal to 4 gallons per stamp ^ .V.* . 






50 STAMP MILLING OF GOLD ORES. 

per minute. Each Triumph takes 1| gallons per minute of 
water, in addition to the battery water accompanying the 
pulp. From 40 to 50 pounds of mercury are consumed in 
crushing from 1,400 to 1,500 tons of ore. This is equivalent 
to 9 dwts. troy per ton of ore treated. 

In leaving the mill the tailings are passed through an auto- 
matic sampler which takes six samples per hour. It is the 
invention of Mr. Starr, the former superintendent, and 
f'>^ / ' ' appears to be an excellent device. The results indicate that 

the mill saves from 85 to 87 per cent, of the gold in the ore. 

The tailings rarely contain less than $1 per ton. 

. The W. Y. O. D. (Work Your Own Diggings) is a new mill, 

^ ^ ^^'^ having been erected in 1890. It contains 10 stamps weighing 

l\}i\' 750 pounds each. The speed is regulated at from 90 to 100 

^tui drops per minute. The drop varies from 5 to 7 inches. The 

^ ; depth of discharge has a minimum of 4 inches and a maximum 

I ^ of 6 inches. The dies, which are made of cast iron, are used 

AiicL^ for six weeks and then discarded. This prevents a wide diflFer- 

C ence in the issue. Chrome steel shoes are used and give 

excellent service. 

The mill is provided with 1 rock-breaker (Gates) and 2 

automatic feeders (Hendy Challenge). The crushing* capacity 

is at the rate of 510 tons per month of 30 days. This is the 

only mill in Grass Valley which works on Sundays. 

About one-half of the amalgam obtained from the inside of 

'^/.jj^ J the mortar is taken from the one amalgamating plate, which 

^ is 4 feet long and 4 inches wide. Of the total saving of the 

mill by amalgamation two-thirds comes from the mortar. The 

tendency is to endeavor to increase the importance of this 

r'-^f* inside amalgamation. The amalgamating table consists of 

"^ one surface 50 inches wide and 14 feet long. It has a slope of 

1^ inches per foot and is plated with 5 ounces of silver per 

square foot of copper. 

' Brass wire-cloth screens of 40 mesh have been replaced by 

" ^ ^'-^ '^vA perforated tin plate. The latter give a service of 14 working 

days. They are considered to wear more uniformly than the 
wire cloth, the brass of which becomes amalgamated at the 
^ edges, and so weakened. 

•A><>^,K^ The pulp from the table goes to 4 Frue vanners, which 
^ extract 2^ per cent, of concentrates worth from $60 to $275 per 






TYPICAL STAMP MILLS OF CAUFOBSIA. 51 

ton, the average being about $100 gross. They are sold to the 
local chlorination works, which charge $16 per ton for treat- 
ment and hauling. About 90 per cent, of the gold value is 
returned, the silver being negligible. 



The Union MUl, San Andreaa, Cal. 

Three flasks (each 76^ pounds avoirdupois) of mercury are il(^ 
consumed in crushing 6000 tons of ore. This is equivalent to -y 

11 dwts. per ton. The water used in the mill amounts to 2i ^*^'-i..J ' 
miner's inches, being at the rate of 3 gallons per stamp per Luj^ ''- 
minute. ^-^^ ^ 

The mill is run by steam-power. The plant is small, but 
very sensibly designed. The failings are eaid to contain an 7^,^- 
average of $1.10, being equivalent to a saving of from 87 to 90 ^•'^i-,— 

per cent. 3 

The Idaho mill is a very old plant, belonging to a famous ^'-^ 

old mine, and has undergone frequent alterations. The figures ' 

given in the comparative table indicate that it works on lines 
similar to the Empire mill. 



52 STAMP MILLING OF GOLD ORES. 

Before concluding this description of the mills of Grass 
Valley, it will be well to draw attention to one or two matters 
of detail. 
-i-. Depth of Discharge. — At the North Star there is no serious 

effort made to maintain a uniformity in the depth of dis- 
charge. This is a mistake. A certain depth of discharge will 
give the conditions most favorable to good work with a certain 
kind of ore; it should be the business of the millman to 
ascertain what this particular depth is, and it should then be 
his effort to prevent too wide a variation from it. At the 
Empire this is done in the two ways previously described, 
while at the W. Y. O. D. mill the dies are only used for six 
weeks and are then discarded. 

The plan adopted in the latter case is a sensible one because, 
though there is a waste of iron in discarding dies that are not 
quite worn out, this loss is lessened by the sale of the scrap, 
and is offset by the prevention of that excessive depth of 
discharge which occurs when dies are allowed to wear down 
to the bitter end, and consequently seriously diminish the 
crushing capacity of the battery. 
-/■ Screens, — In the matter of screens the millmen of Grass 

Valley have taken a retrograde step. The adoption of round- 
punched tin-plate screens in place of wire cloth does not 
appear to the writer to be any sort of an improvement. Many 
years ago the ordinary round-punched Russia iron gave place 
to angle-slot screens for the reason that it was found that the 
round-punched iron was constantly becoming clogged. The 
openings of the angle-slot screen kept themselves more clear. 
In time it was found, however, that the angle-slot screen 
was unsatisfactory, because the sizing which it did was, to a 
marked degree, irregular. Brass wire cloth was introduced. 
The time of service of the angle-slot was nearly twice as long 
as that of the wire cloth, but this was offset by their cost, 
which was also twice as much. The former cost $3.90 per 
screen and the latter $1.94. The brass wire was made of 29-30 
and 30-31 wire. Steel wire cloth was also tried, but while it 
wore well — that is to say, it was not quickly abraded — yet its 
usefulness was much diminished by the fact that the hori- 
zontal wires shifted, presenting the appearance shown in the 
accompanying sketch, Fig. 4, where it is evident that at hh 



TYPICAL STAMP MILLS OF OALIPOEJfLA. 53 

there will be scarcely any discharge, while at a a coarse pulp 
can readily find an exit. Therefore they were discarded and 
brass wire cloth, which did not share this defect, was 
substituted. 

The average life of a brass wire screen, 30 mesh, was found 
to be from 20 to 24 days, that is to say, it served to discharge 
about 200 tons of crushed ore. During a period of 4 years 
there were crushed at the North Star mill 70,000 tons of ore, 
and the expense in screens during this period amounted to 
$675. The screens were made to order in rolls 100 feet long 
and of such a width, 45 inches, as permitted of three strips 
being cut. Each strip was 15 by 52 inches. One roll of brass 
wire cloth cost $135, so that the screen cost $1.94. The 
expenditure under this head was therefore slightly less than 
1 cent per ton of ore treated. 



Recently, punched tin-plate screens have been introduced ; 
and as is so frequently the case in matters of this kind their 
adoption by one plant has led to their use at nearly all the 
Grass Valley mills. It is often the practice here, as it used to 
be at Angels Camp, Calaveras, to burn off the tin upon the 
blacksmith's forge, with the idea that in this way the iron 
plate became annealed and therefore toughened. Since the 
tin will amalgamate, its removal also prevents the adhesion 
of mercury. When the tin is not removed as above described, 
it is found that at the end of the first week's service the 
abrasion of the pulp has rubbed off the tinned surface. In 
this respect tin plate is no improvement on brass wire. It is 
true that the brass becomes partially amalgamated and the 
screen rots in consequence ; but previous to the time when 



54 STAMP MILLING OF GOLD ORES. 

this action has become of any importance, sharp bits of 
quartz have cut the wire, and it has become necessary to make 
a patch and to turn the screen frame upside down. Mr. 
Abadie informed me that he found that the brass wire had a 
shorter life when the stamps were crushing waste — that is, 
wall rock containing only a very small amount of gold — and 
very little amalgam was being discharged through the screens, 
than when good gold quartz was being treated and more mer- 
cury was being added to the millstufF. 

Brass wire screens now cost $1.55 each, which is about three 
times as much as the tin plate, which costs only 55 cents per 
screen ; the former gives a service of 25 days, as against 14 to 
15 days for the latter. But the relative expense is not worth 
notice since per ton of ore it is less than 1 cent. As compared 
to angle-slot or round-punched screens the wire cloth has the 
immense advantage that, with openings permitting of the 
passage of particles of equal size, it has a much larger area of 
discharge per square foot of screen. It therefore gives a 
more even product and produces less slime. A screen is a 
device for sizing the particles of pulp to be delivered to the 
amalgamating or concentrating apparatus placed below the 
stamps. Wire cloth most nearly gives the sizing medium re- 
quired, while angle-slot or round-punched iron plate only very 
imperfectly fulfills this function. 

All the Grass Valley mills have their screen frames at a 
forward inclination of about 10 degrees. As compared to a 
vertical arrangement, this helps to spread the pulp over the 
surface and to give the aid of gravity to the force of the 
splash in ejecting the crushed ore. 
-^ Amalgamating Plates^ — It will have been noted that the ten- 

dency at Grass Valley is to increase the amount of silver put 
into the electroplated copper amalgamating plates. At the 
North Star the plates carry 1 ounce of silver per square foot 
of copper ; at the Empire, ^\ ounces ; and at the W. Y. O. D., 
the newest mill, the quantity has been increased to 5 ounces 
per square foot. 

The milling practice of this typical Californian mining dis- 
trict has undergone one important modification during recent 
years. I refer to the introduction of an inside amalgamating 
plate. This was done at the North Star mill in 1888. It was 



TYPICAL STAMP MILLS OF CALIFORNIA. 55 

found that the shallow discharge brought the plate too near 
the stamp ; therefore the chuck-block was inserted. The latter 
gives a sort of false lip to the mortar box and serves to 
heighten the discharge. It is a wooden block, extending the 
whole of the front of the inside of the battery, and to it the 
plate is attached. (See drawing, Fig. 1, of mortar at North 
Star mill.) At the North Star and W. Y. O. D. mills twice 
as much amalgam is saved inside the mortar as upon the 
tables outside, while at the Empire from 50 to 86 per cent, of 
the total amalgam obtained by the mill comes from the inside 
of the mortar. Of the saving thus effected in the battery, 
about one-half comes from the plate itself and the other half 
is derived from the treatment, at the fortnightly clean-up, of 
the battery residues.* The tendency at Grass Valley is to 
increase the percentage of saving effected within the mortar. 
This feature of the milling practice is of great interest, 
since it indicates a tendency to make the mortar more of an 
amalgamating machine than has heretofore been the case in 
California. On the Pacific slope the stamp battery is 
primarily a crushing machine, hence the quick, short drop 
and the shallow discharge. The work of amalgamation has 
hitherto been left for the most part to other apparatus, and 
the introduction of an inside amalgamating plate tends to 
bridge over the wide divergence existing between the milling 
methods of Colorado and California. The modification is 
right in principle to the extent that the sooner you catch your 
gold the less likelihood there is of loss, and if you can extract 
it in the mortar, do not allow it to pass out in order to arrest 
it on the tables. On the other hand, the cheap milling of 
California is largely dependent upon the rapid crushing of the 
ore by the fast-dropping stamps ; and the use of an inside 
plate, by demanding an increase in the depth of discharge, 
compels a diminution in the crushing capacity of the mill. It 
becomes a business proposition to be carefully weighed by the 
manager of the mill, who has to decide whether the increased 
extraction in the mortar more than compensates for the less 
rapid reduction of the ore. The best practice is that which 
pays best. 

^^■i"^-^— i— ^-^i^—  .1  I I M^ I , -■ . I I . M . »»■■»■■■  .■■■■■■- I  .  , i» — 

* In the Gilpin County miUs all the amalgam saved inside is obtained from the 
two front and baok inside plates. 



56 STAMP MILUNG OF GOLD ORES. 

The little W. Y. O. D. mill sets a good example to its larger 
and more pretentious brethren in the matter of the arrange- 
ment of its amalgamating tables. These are 50 inches wide 
and 14 feet long, giving a clear, wide, full amalgamating sur- 
face, which shows much good sense and judgment compared 
to the short aprons and narrow sluices of the other plants. 
The narrow sluice-plate of the Californian mill is a relic of 
the days when gravel-mining apparatus was introduced into 
quartz-crushing mills ; it is indefensible, and the sooner it is 
discarded the quicker will there be removed the only serious 
blemish of the typical American mill. 

The accompanying drawing will illustrate a representative 
Californian plant. 

Grass Valley has been a school to many good millmen, and 
it has been the birthplace of many of the most important 
improvements introduced into gold-milling practice. At 
another time the writer hopes to trace the gradual growth of 
the American stamp mill from the rough and clumsy machine 
of the early fifties to the magnificent and complete mechanism 
whose muffled thunder now echoes among the foothills of the 
Sierra Nevada. 



CHAPTER IV. 

Oalifornian Practice in Amador County. 

Amador County lies extended among the rolling foothills of 
the Sierra Nevada, sloping from the pine-clad mountains down 
into the dusty valley of the San Joaquin. The county has its 
greatest extent east and west, but from the Cosumnes River 
on the north to the historic Mokelumne on the south it covers 
17 miles of the main gold belt of California. It is a mining 
district whose record fills some of the first and most inter- 
esting pages of the history of Californian gold discovery. As 
early as 1848 mining was begun in Hicks Gulch, a tributary 
of Sutter Creek. The story of that early mining is all of 
gulches and of creeks, for the diggers followed the running 
stream and had not yet realized the importance of the quartz 
lodes. In 1851 a stamp mill was erected on Amador Creek. 
This date also marks the commencement of exploration on 
that great auriferous belt which extends through several 
counties, and has become known as the " mother lode." In 
1856 there were 20 stamps working ore from the Keystone 
mine, 38 on the Spring Hill, and 20 at the Amador mine. 
Motive power was obtained from the creek by using large 
overshot wheels. The gravel deposits of the mountain streams 
became exhausted about the year 1860, and quartz mining 
received a fresh impetus. In 1866 there were 296 stamps 
crushing the produce of 15 mines. In 1874 the number had 
increased to 335 stamps, supplied by 13 mines. It is evident 
from these figures that while the milling capacity had grown, 
the number of mines which afforded the ore supply had not 
increased. The explanation is to be found in the construction 
of the Amador canal, which by conveying water-power to the 
district enabled a very considerable enlargement of the mills 

(57) 



68 



STAMP MILLma OF GOLD ORES. 



to be carried out, and made this district one of the most busy 
and prosperous of the gold-mining regions of California. In 
1879 the Amador Canal Company owned 45 miles of main 
ditch. The total length of canals and ditches was no less than 
94 miles. The water is delivered to the mines under a head 
varying from 130 feet to 470 feet. 

In 1878 there were 610 stamps at work in Amador County, 
crushing the ore of 22 mines. In 1890 the number of stamps 
was almost exactly the same, 511, distributed in 18 mills, 
treating the ore of 18 mines. 

COMPARATIVE TABLE. 



Name of Mill. 



Gover 

South Spring Hill 

Wildman 

Keystone 

Kennedy 

Clinton 



• 

P. 
£ 

S 

CD 
«M 
O 

• 

o 

^ 

20 
40 
SO 
40 
40 
20 


• 




• 

d 
B 

p. 

OQ 

o 
Q 


• 
P4 

2 



w 


Depth of dis- 
charge. 


Capacity per 
stamp. 


Lbs. 




In. 


In. 


Tons. 


800 
750 
750 
750 
850 
1000 


96 
93 
92 
90 
83 
85 


6 


8 

f 

7 
1% 


2.2 
2.8 
2.8 
2.5 
2.5 
8 



•IH * 

p4d 



Tons. 

45 

92 

65 

100 

100 

60 



Kind of screen. 



Gover— Several varieties 

South SprineHill— No. 8 angle-slot 

Wildman— No. 6 angle-slot 

Keystone— No. 8 straight slot . . . . 
Kennedy— 80-mesh brass wire . . . 
Clinton— 20-mesh brass wire . . . . 



• 

0) 

£ 

o 
CO 

O 

S3 


Percentage of 
concentrates. 


Value of concen- 
trates. 


1 

2 
Pi 

»- -; 

4^ bo 
V 03 

87 
45 
42 
45 

42 
80 


Fineness of 
bullion. 


Loss of mercury 
per ton of ore. 


Days. 




Dol- 
lars. 


Per 

1000. 


Dwts. 


45 
84 
86 
82 
80 
25 


1 
\% 

2 ^ 
2 


110 
40 
60 
110 
125 
100 


825 
880 
809 
880 
828 
820 


2.6 

* 

4.5 

6.5 

* 

* 



p< 

J® 



03 



CO 



Gals. 



8^ 

3 

8^ 

8 

6 



During 1892 the State of California produced gold having a 
value of $12,571,900. Of this total Amador County yielded 
very nearly $1,500,000. At the present time the district is 
fairly prosperous ; some of the old mines have recently been 
reopened, and there is reason to expect that there will be a 
renewal of that activity which characterized the county a 
few years ago. In 1895 the production was $1,391,929. 

♦Figures not obtainable. 



CALIFORNIA^ PRACTICE IN AMAI>OR COUNTY. 59 

The foregoing table gives the chief figures indicative of the 
methods of milling at five of the best-known mills. They 
are scattered among the series of picturesque little mining 
towns which reach from Drytown to Mokelumne Hill. Most 
of the plants are old, but have been variously modified so as 
to meet the changing requirements of the ores. 



iDterlor of the Gover Mill, Amador Couotr, Cal. 
Of the five mills whose names appear on this list I shall 
endeavor to give a detailed description of one, and that one 
will be the Gover mill, which in many ways fairly typifies the 
best practice of Amador County. This plant is located about 
two miles north of the town of Amador. The mill, the mine, 
the tramway, which connects them and the surrounding hill- 
slopes with their clumps of white oaks, compose a picture 
thoroughly characteristic of the mining regions of the Cali- 
fomian foothills. The accompanying photograph will there- ' 
fore prove of interest.* The Gover mill is 19 years old ; it is 
a comparatively small plant and consists of only 20 stamps. 
The weight of each stamp is now about 800 pounds, though 



60 STAMP MILLING OF GOLD ORES. 

^ I when new the figure was 850. The speed is regulated at 9ft 

^ ^f^^uj;^^ drops per minute. The height of the drop is changed with 

c \ ^'1 the hardness of the ore to be crushed, the minimum being 6- 

- inches and the maximum 7| inches. The capacity of the mill 

"^ > i ^ shows a wide variation^ according as the ore is hard or soft, 

rv free from sulphide minerals or rich in them, carrying the gold 

^*Mc'J,\'^ • in a coarsely divided condition or containing it intimately 

mixed with pyrite. The minimum capacity is 40 tons and the 
maximum 85 tons per day. The average is about 55 tons or 
2| tons per stamp per 24 hours. At the present time the ore 
is of high grade and an endeavor is made to retain the gold 
inside the battery ; the crushing capacity is therefore some- 
what sacrificed, and averages 45 tons per day. On the other 
hand, last year when the ore was low grade and valuable 
more for the gold-bearing pyrites than for its free, readily 
amalgamable gold, the mill was used more as a crushing 
machine, preparing the millstufF for concentration, and at 
that time 2,600 tons per month were ordinarily treated, being 
at the rate of over 80 tons per day or about 4 tons per 
stamp. 
Q:^ Several kinds of screens are used, in accordance with the 

^^ ^AA^ character of the ore being crushed. Brass wire cloth, 30 

mesh, is employed for ore of medium hardness, carrying gold 
of medium fineness. Such a screen lasts for from 35 to 55 
days. For ores rich in pyrites, steel wire cloth, 20 mesh, is 
generally used. It gives a rapid discharge and wears from 20 
to 30 days. When the ore contains the gold in a fine state of 
subdivision and, as usually happens, is also very hard, then 
punched Russia iron, size No, 7, is employed. This tends to 
keep the pulp inside the mortar and to give fine crushing. 
Such screens last from 40 to 60 days. 

In every instance the wear of the screen is most irregular. 
This is due in large part to the chips of wood (from under- 
ground timbers) which get stuck in the screen openings and 
make frequent scraping necessary. Incidentally I may men- 
tion that by actual count I found that the No. 7 Russia iron 
screen contained 10, the 30-mesh brass wire 23, and the 20- 
mesh steel wire 18, holes per linear inch. The brass wire 
gave much the most free discharge and the most even crush- 



CALirORNIAN PRACTICE IN AMADOR COUNTY. 61 

ing, the steel the coarest crushing and the most rapid dis- 
charge, and the punched iron the most uneven crushing and 
the most irregular discharge. 

From the battery the pulp passes over a series of amalga- /) 
mating tables, to be shortly described, and then to the con- ^^u^^ 



Uj 




iiMlC^ 



centrators. These last are 4 Woodbury machines, constructed 
on the principle of an endless rubber belt having 13 subdi- 
visions. The partition into narrow strips is supposed to give 
a greater capacity than the plain belt. It does less clean 
work, however. Ordinarily at this mill the yield of concen- 
trates is equivalent to 1 per cent, of the ore crushed. In 1891 
650 tons of sulphurets were obtained during the crushing of 
22,400 tons, being in the proportion of about 2| per cent. At 
that time the assay value of the concentrates was about $100 
per ton ; now it ranges betwen $100 and $125 per ton. They 
are sold to the chlorination works at Amador City and at 
Drytown. 

The water used for milling purposes, inclusive of the con- 
centrators, amounts to 10 miner's inches, or 108 gallons per 
minute, when treating ore heavy in pyrites, and diminishes to ^^f( 
6 miner's inches, equivalent to 65 gallons per minute, when 
ordinary quartz is being crushed. Immediately outside the 
mortar there is a pipe for distributing clear water. As a rule 
more water is wanted on the plates outside than is required 
inside the battery. When the reverse is the case the clear 
water is diverted inside so as to make up the requisite 
quantity. 

The consumption of mercury depends upon the richness of 
the ore. While the gold of rich ore is also usually coarse and 
requires proportionately less mercury than the gold which is ^( . / 
in a fine state of subdivision, and which therefore becomes M^ 

more thoroughly amalgamated, yet in practice it is found that 
the consumption of mercury per ton of ore is greatest with 
the richest ores because they require the more free addition 
of mercury, and therefore the greatest opportunity of loss by 
subdivision of the mercury when in the battery. The pres- 
ence of pyrite is also an important factor. The friable, finely 
divided particles of pyrite coat the globules of mercury and 
prevent them from coalescing. In the treatment of 22,400 
tons of ore in one year, 200 pounds of mercury were con- 




'^ 



^Mfr-U, 



62 STAMP MILLING OF GOLD ORES. 

sumed, being at the rate of 2.6 dwts. troy per ton. During 

the week of my last visit the ore being crushed was of high 

grade, and 60 ounces of mercury were being used per 24 hours. 

It is added to the ore in the battery by the amalgamator at 

[{j^^^ regular intervals in a small wooden spoon, and most of it is^ 

A^ivJ of course, recovered when retorting the amalgam. Thus, in 

retorting 350 ounces of amalgam there were recovered 191 

ounces of quicksilver. The amalgam is now worth $7.50 per 

ounce, but usually averages $6.55 per ounce, equivalent to a 

retort yield of 37 per cent. gold. 

The gold saving is done directly in the mortar itself, and 

^ &y^l ^^ *'^® amalgamating tables outside ; also indirectly by the 

concentrators and a system of blankets which follow. As 
already stated, free mercury is added to the ore as it is fed 
into the battery. This serves to collect some of the gold 
which then settles as amalgam to the bottom of the mortar^ 
being deposited in the corners and between the dies. The 
work of gold saving is, however, supplemented by the use of 
an inside copper plate. This is placed at the front of the 
mortar, immediately under the screen frame. It is fixed to a 
wooden block called the "chuck-block," whose inside surface 
is curved and carries the copper plate, while the front fits 
tightly against the lip of the mortar. The plate is 6 inches 
wide and the full length of the interior. The depth of the 
Kui^ chuck-block is 8 inches, and it serves to heighten the issue by 

raising the front of the mortar. It has been found that a 
curved chuck-block (see Fig. A) gives a more rapid discharge 
than a straight one (see Fig. B), such as was in use formerly 
at the Plymouth Mill. 
Of a total clean-up of 350 ounces of amalgam, 277 ounces, 
ijj^ or 79 per cent., were obtained from inside the battery, and 73 
\ ounces, or 21 per cent., were scraped from off the outside amal- 
, gamating tables. Of the total obtained inside the battery, 

'; LuK^ 221 ounces, or 63 per cent, of the whole 350 ounces, came off 

the plate, and the balance of 56 ounces was obtained from the 
treatment of the sands remaining inside. 
The pulp as it issues from the mortar is discharged upon a 
<^^Lf uj series of amalgamating tables having various dimensions. The 

screen surface is small ; it is only a strip 4 inches wide by 4 
feet long. This is one reason for the comparatively long 






ft^nK 



VAxiu>^- 



/^ J 



\ 



CALIFORMIAN PRACTICE IN AHASOR COUNTY. 63 

service given by the screen, and as the discharge of a mortar 
is nearly always confined to the bottom part of the screen sur- 
face, it is an arrangement which does not, I think, seriously 
interfere with the rapidity of the issue. 




The accompanying sketches illustrate the arrangement of the 
amalgamating tables. The pliotograph of the interior of the 
mill will also help to make the description clear. Oo issuing 
from the mortar the pulp falls upon an amalgamated plate. 



64 STAMP MILLING OP GOLD ORES. 

called the battery plate. This is 4 feet 2^ inches wide and 
19 inches deep. It discharges in turn upon another plate, 
called the " splash-plate," arranged at an angle of about 45 de- 
grees, and which is 8 inches deep and 46 inches wide. Then 
the pulp runs over the "apron," a plate 3 feet deep and 50 
inches wide. The arrangement just described (and illustrated 
in Fig. 0) serves to give an extra amount of amalgamating 
surface without taking up much space. From the apron the 
pulp passes into a small transverse trough, and then goes 
through two openings, which deliver it to the " distributors," 
which in turn allow it to pass through on to the sluice-plates. 
(See Fig. D.) The " distributor " consists of a copper plate 
punched with holes 1 inch apart, and serves to break up the 
pulp and to scatter it evenly over the plates which follow. 
The sluices have a slope of 1^ inches per foot and the apron 
li inches per foot. The former are 14^ inches wide and 11 
feet long. They deliver their burden to another, a single 
sluice, 14 inches wide and 6 feet long. From the last the pulp 
goes through a well or mercury trap before passing over a 
launder for delivery to the concentrators. 

The whole arrangement of the plates is lacking in common 
sense and is most unpractical. That it is scarcely defensible 
is fully recognized by the manager, who had to take the mill 
as it was, but who intends to have a very different arrange- 
ment in the new plant which it is proposed to erect. The bad 
arrangement of the amalgamating tables is a universal defect 
of the mills of this region, and will be again discussed later on. 

The general clean-up takes place once a month. The inside 
(^Jj ^ ^ plates are then taken out of the mortar and scraped. The 

'^ \U battery sands are treated in a barrel where the grinding is 
* assisted by the addition of scrap iron. After 8 or 9 hours 
quicksilver is introduced, and the charge is retained for 4 
hours more. The aprons and splash-plates are dressed when- 
ever their surface becomes hard. Occasionally very finely 
divided sulphurets collect and have to be brushed off. When 
crushing such ore as is comparatively free from valuable 
pyrites and containing gold that is easily caught, the mortar 
will arrest over 90 per cent, of the total obtained by the entire 
mill. The aprons and splash-plates are then only cleaned up once 
a month, and the sluices once a week. When treating sulphuret 



i^C 



CALIFORNIAN PRACTICE IN AMADOR COUNTY. 65 

ore, — that is, millstuff containing 1^ to 3 per cent, pyrites, — ^^ 
about 70 per cent, of the total saving is accomplished inside ^^^[ 
the mortar, and of the remaining 30 per cent, half is caught ^ 

by apron and splash-plate, half by the sluices. When crush- 
ing such ore the aprons are cleaned off partially every other 
day and the sluices daily. 

From the concentrators the waste goes outside the mill 
proper to another building, where it passes through a tailings ^ / 
plant. The tables are in 8 sections, having a total width of ^^ 

115 feet and a depth of 18 feet. They are covered with thin 
canvas, that variety known as 10-ounce Arctic duck. Before ^ 
flowing over the tables the pulp is roughly sized by means of ^^^^^ . 
settling boxes; the heaviest material then goes to the four ^ V/ 

central divisions. Below the tables there is a series of setling 
pits which is cleared up at intervals of from two weeks, the 
uppermost, to six months, the lowermost. The "heads" are 
shoveled into a box and washed by a running stream on to a 
Woodbury concentrator, having 11 partitions. The concen- 
trates thus obtained are found to be in an extremelv fine state 
of subdivision. To prevent loss by their suspension on the 
water running away, care is taken to keep the concentrates 
under water as much as possible. They can be seen to float 
in the receiving box under the concentrator belt, but from 
here they are run through a pipe which issues under water in 
a tank lower down, where they settle. A saving of $15 per 
month has been made by the prevention of loss through sus- ^ 
pended slimes. The tailings plant gives one-quarter of the ^t(^/ 
total yield of concentrates ; thus, of the last 15 tons obtained, ^'^^v 

3^ tons came from here, and 11^ tons from the upper mill. The 
value per ton varies, usually those from the tailings plant are 
the richest, but occasionally they are poorer than the average C 
of the upper mill. This plant requires one man's labor ; he ^ ^^^^^i^ 
is paid $75 per month. The canvas costs at the rate of $15 
per month. 

Iron shoes and dies are used at this mill. Good and cheap 



/ 



a 



castings are obtained from the neighboring foundry (Knight's) ^< ^ 

at Sutter Creek. One reason given for the preference for ^^t^ 

iron over steel is that the latter wear longer, and therefore 
too much amalgam is locked up among the dies during the 
period which elapses before they are changed. In the crush- 




CALIFORNIAN PRACTICE IN AMADOR COUNTY. 69 

discharge less than the height of the drop, the former being 
5 inches and in the latter 6 to 7 inches. It was found that 
when the dies became worn (and the issue thereby deepened) 
more gold was saved inside the battery. It became desirable 
to perpetuate such favorable conditions. The lip of the 
mortar was therefore heightened by the device known as the ^ f 
*' chuck-block," a wedge of wood extending along the whole Hf/ ^^ 
length of the front interior of the mortar. The discharge >^ ^ 

being thus increased the stamp worked in a greater depth of ^^ttrj^ 

water, the splash became much less violent, and conditions ' \ 

were obtained which rendered possible the use of an inside 
amalgamating plate. Previously a plate could not be 
employed to any purpose because the shallowness of the ^,^, 

mortar compelled it to be placed so near to the dies that its 
surface was continually being scoured by the violent splash of 
the pulp. Thus from the purely Oalifornian type the Amador 
mill has made an approach to the Gilpin County (Colorado) 
pattern. However, even in the case of the deep discharge 
(13 to 15 inches) and long drop (16 to 18. inches) of the 
Colorado mill the latter exceeds the former, and the stamp is 
lifted entirely out of the water. 

The question here presented is a most interesting one. 
When the stamp strikes the surface of the water in the 
battery there is a certain resistance offered by the passage 
from air to water. This is largely a theoretical consideration, 
but in practice the chief effect produced by not lifting the 
stamp free of the water is to diminish the force of the dis- 
charge. Instead of a violent, irregular splash there is pro- 
duced a pulsating, wave-like movement, which results in 
making the issue of the pulp through the screen less violently 
intermittent, but more continuous, and it allows a better 
opportunity for the settling of the particles of gold upon the 
inside plate and to the bottom of the mortar, ^ 

The use of the chuck-block (see drawing of a typical Amador ^ ![,, / 
mortar) also enables the millman to regulate the issue and to '^ 

prevent, to some extent at least, that wide difference between I ^/^^ 
maximum and minimum depth, which is due to the wearing ^ v 

down of the dies. At the South Spring Hill two chuck-blocks 
are used, one being 6 inches and the other 7 inches high. 
The former replaces the latter as the dies get worn down. 



70 STAMP MILLING OF GOLD ORES. 

When the dies have lost 3 inches, a false bottom, made of steel 
and 3 inches thick, is placed underneath them, thus restoring 
the former height of the issue. At the Wildman mill an 
endeavor is made to maintain a uniformity in the discharge 
by using chuck-blocks of various thickness, from 4 to 7 inches. 
At the Kennedy the chuck-block is made in sections, bolted 
together. The uppermost part, which carries the copper 
plate, is 2 inches thick ; the others are each 1^ inches. The 
maximum height is 7 inches. As the dies wear down the 
discharge is maintained as uniform as possible by removing 
sections of the chuck-block. 
^. Amalgamation, — ^The proportion of amalgam saved inside 

' the mortar as compared to that which is arrested on the 

plates outside will, of course, differ with the changing char- 
acter of the ore treated. It will also depend largely upon the 
depth of discharge and the kind of screen in use. A deep 
issue will tend to arrest the gold inside the mortar at the 
expense of the crushing capacity. The same result will be 
produced by a fine screen. 

fi I ,, . At the South Spring Hill about 55 per cent., or rather more 

^%vv\y than one-half of the amalgam, comes from the inside of the 

mortar. No blankets are used at this mill. The tailings are 

sold to outside parties for $15 per month and 15 per cent, of 

the gross yield. 

At the Wildman more than three-quarters, or from 80 to 83^ 
per cent., of the total amalgam is arrested inside the mortar. 
At the Kennedy about three-quarters of the amalgam comes 
from the clean-up of the inside of the battery. 
L Concentration, — It is the usual practice in Amador, as else- 

where in California, to use two belt concentrators for each 
five stamps. The Frue vanner is most commonly found in the 
mills. At the South Spring Hill three kinds of concentrators 
are used, 8 Frue vanners, 2 Woodbury, and 2 Triumph 
machines. The last mentioned are said to require very 
frequent attention. The Frues are preferred. 

This mill produces nearly 1^ tons of concentrates daily. At 

(\ ^ the Wildman mill there are 8 Frues and 4 Triumphs. The 

^^tryjJK^former are preferred because they require less frequent atten- 
tion. This plant yields 1 ton of concentrates daily. At the 
Kennedy there are 18 Frues, or two machines more than the 



CALIFORNIAN PBACTICB IN AMADOR COUNTY. 



71 



-^ 



ordinary allowance of two concentrators to five stamps. The 
addition of the extra vanners has proved most satisfactory. 
The Frue is a machine you can not crowd, and the distribu- 
tion of the concentration over three, instead of two, machines 
allows of better contact between the belt and the pulp. This 
plant produces from 60 to 72 tons of concentrates per month 
as the result of crushing from 2,900 to 3,150 tons of ore. This 
is at the rate of about 2^ tons of concentrates per day. 

The Amalgamating Table, — In an earlier portion of this 
chapter reference was made to the bad arrangement of the 
amalgamating tables which prevails in the mills of this as in 
those of most of the Californian gold-mining districts. The 
following figures will indicate this feature : 



MiU. 



Gover 

South Spring Hill. 

Wlldman 

Kennedy 

Keystone 



Aprons. 



be 

d 
a> 



8 ft. Oin. 
2 " " 
2 " " 

2 " 6 " 
10 



« 






4 ft. 21^ in. 

3 " 10 " 
8 " 9 " 
8 " 10 " 

4 « ^ a 



ft . 



l^in. 
1 " 



Sluices. 



.d 
a 



lift. 
16" 

14 " 
10 " 
10 " 



A 
•d 



29 in* 

26 " 
28 " 
28" 

30 " 



ft . 



J" 






It will be seen that in each case the wide apron-plate, 
placed immediately outside the mortar, has a short length 
and is succeeded by two narrow and long sluice-plates. The 
sense of this arrangement is not obvious, but it is rarely 
questioned and is almost universal in the mills of California. 
It is only another illustration of "that everlasting monkey 
in man," by virtue of which Jones copies Thompson's mill 
because Thompson erected his mill after the pattern of Smith's. 
Smith's was considered the best mill in the country, for, did 
not his mine turn out a bonanza, and was not the machinery 
built by a well-known firm in San Francisco ? 

It is a serious matter, however, that the stamp mills of Cali- 
fornia, otherwise easily the best of those in use to-day, should 
be spoiled by such an obvious error. The gold caught on the 
apron consists of the heavier particles whose weight prevents 
them from traveling further down the tables ; the finer par- 
ticles are not so easily arrested, but are supposed to be caught 

* In every instance subdivided into two equal divisions. 



72 STAMP MILLING OF GOLD ORES. 

upon the sluice-plates. That which is saved on the apron 
— mostly in the first foot of its length — can hardly escape, 
but that which is arrested by the sluice-plates is obtained 
in spite of, rather than by reason of, the methods adopted. 
The quantity of pulp which passes over a surface 4 feet 
wide is made to pass over another surface 28 inches wide, the 
water and crushed ore are crowded into one-half the space and 
therefore must flow at twice the speed. It is proposed to save 
gold particles more finely divided than those already caught by 
the apron, but under conditions a great deal more unfavorable 
to amalgamation. One would naturally suppose that the sluice- 
plates would be placed at a less slope than the aprons, so that 
the flow of the pulp might be moderated. It is not so ; in 
most cases the inclination of the sluice-plates is steeper than 
that of the aprons. The former, therefore, act as launders to 
convey the pulp to the concentrators, but it is absurd to con- 
sider them as intelligently planned gold-saving appliances. 
The California stamp mill will always be characterized by a 
very serious blemish while it retains so unscientific and so 
unpractical an arrangement of its amalgamating tables. 
In concluding this description of the mills of Amador 
A County, it will not be out of place to refer to the general 

-^^V/^ character of the work done in the district. The costs of 

mining and milling are uniformly low, but yet not quite so 
low as the figures often quoted would lead one to suppose. 
It is not unusual to see the assertion made in a prospectus or a 
newspaper to the effect that the total cost of extracting the 
gold from the ore, including not only mining and milling, but 
also general expenses, interest on machinery, repairs, etc., is 
$2.50 per ton, or even less. One of the largest mining con- 
cerns ever operated in this district is the Plymouth, a mine 
1,600 feet deep, which paid several millions in dividends. In 
1887 the average yield was $7.57 per ton. In 1888 it was 
$6.18. The cost of mining was $2.34, of milling 39 cents, of 
concentration and reduction of sulphurets 17 cents, and gen- 
eral expenses 17 cents, making a total of $3.07 per ton. This 
is a fair instance of what can be done by a large mine in this 
district. In the case of three representative mining compa- 
nies now working profitably with mills of 20, 40, and 30 stamps, 
respectively, the total costs are $2.35 to $3.45, $3.50 to $3.75, 









.g 







•cr 



"^.fr' 



wwv 






1®/ 




V^AAv 






Section of an Amador Stamp Mill. 



74 STAMP MILLING OF GOLD ORES. 

$3.10 to $3.25, per ton of ore. When once a mine is devel- 
oped to the extent of having enough ore in sight to supply a 
40-stamp mill for one year it may be taken that the total cost 
per ton, including a proper proportion of exploratory dead 
work, interest on plant, repairs, etc., need not be over $3 per 
ton. The cost of mining would be $1.75 to $2.25, of milling 
20 to 35 cents (not including water-power, or 35 to 50 cents, 
inclusive), concentration and chlorination 15 to 25 cents, 
general expenses, etc., 20 to 30 cents. 

It ma}'^ therefore be said, by way of conclusion, that while 
the mills of Amador County are by no means perfect, yet the 
methods of mining and milling pursued in this district have 
done much to build up the reputation which California has so 
long enjoyed for leading the way in the matter of the 
economical extraction of gold from its ores. 



CHAPTER V. 
The Profitable Working of Large Bodies of Low-Grade Ore. 

The first mining excitement occurring in this region is to 
be credited to the year 1875, when placer discoveries were 
made at Custer City, at that day the frontier post and the 
starting-point for prospecting parties. In June of the follow- 
ing year the Wheeler Brothers found rich gravel in Deadwood 
Gulch, between the sites of the present towns of Deadwood 
and Central City. They, and others who came later, cleaned 
up that November, pulled up stakes and sold their claims. A 
party of about 30 men carried the result of the season's labors 
—namely, gold of a value of from $800,000 to' $900,000— to the 
First National Bank at Cheyenne. 

At that time, early in the story of the restless development 
of the Rocky Mountain region, the outcrops of quartz veins 
were found by the pioneers, but the California and Montana 
miners who were among them tested the value of the ore by 
means of panning, and recovering only from $3 to $10 per 
tons they ridiculed the idea of operating on material of so low 
a value. 

On April 7, 1877, a 5-stamp mill for the Racine Company 
arrived at the lower end of what is now Lead City. On April 
20, the first clean-up took place. On May 1 the second stamp 
mill, Pearson's, reached Central City. 

The history of the Homestake Mining Company begins with 
the arrival, in the fall of 1876, of representatives of Haggin 
& Hearst, of San Francisco. In January, 1877, Sam. McMas- 
ters, another agent of the same syndicate, came ; in February 
some of the mines were purchased, and on October 23 the 
Homestake No. 1 claim was bought from Manuel Brothers. 
In July of the following year the first and only assessment 

(75) 



76 STAMP MILLING OF GOLD ORES. 

was levied. It was at the rate of $1 per share, and afforded 
the $200,000 required for the erection of the first mill. This 
was of 80 stamps, and was brought in on a bull train. It now 
forms part of the present Homestake mill. As the magnitude 
of the ore deposit became proved by exploration, the milling 
capacity was increased, until the Homestake became a name 
associated with the successful operation, on an enormous 
scale, of large bodies of gold ore of very low tenor. 

The Black Hills, geologically an Archaean island flanked by 
later sedimentaries, topographically a group of granite peaks 
surrounded by clustering foothills, forms the only important 
mining region of South Dakota. Extensive and important 
mining operations are largely confined to what is known as 
"the Belt," extending from Whitewood Creek to Deadwood 
Gulch, through the linked settlements of Lead City, Terra- 
ville, and Central City. 

The ore occurs in the form of large bodies of quartzified 
/K chloritic schist conformable to the structure of the country 

^\^ and forming a portion of it. The area of territory which has 
been proven profitably gold-bearing is about a mile and a half 
long by half a mile wide. The width of milling ore varies 
from 50 to 400 feet. Huge excavations have been made along 
the outcrop of the lode, the ore going direct to the mill, while 
the waste material is sent down into the mine to fill up the 
underground stopes. 

In 1888 there were 660 stamps dropping on the millstuff 
broken upon the Belt ; to-day the number is 550. The richer 
ore bodies of the De Smet and Caledonia mines have given 
way to material too low in gold contents to be profitably 
worked. The mills of the Homestake Mining Company 
(shown in the accompanying illustration), however, show 
progressive enlargement, as is indicated in the accompanying 
table (No. 1), which gives the names and sizes of the various 
plants in this district. 

Referring to this table, it may be added that the Homestake 
was an 80-stamp mill when first built, and it is still often 
spoken of as "the 80 mill." The Highland is now being 
increased from 120 to 140 stamps. The Deadwood and the 
Golden Terra were consolidated six years ago, the 80 stamps 
of the Terra being placed behind the 80 stamps of the old 



78 



STAMP MILLING OF GOLD ORES. 



Deadwood mill. The plant is now known as the Deadwood- 
Terra. The De Smet ceased work in 1892, and the Caledonia 
in 1893. The Columbus is a new, small plant, treating ore 
from the outskirts of the Belt at Central City. 

Table No. 1. 

THE STAMP MILLS OF THE BELT, SOUTH DAKOTA. 



Name. 


Date of 
erection. 


Homestake . . . 
Golden Star . . . 


1878 
1879 


Highland . . . 


1880 


Deadwood . . . 
Golden Terra . . 


1879 
1880 


Father de Smet . 


1878 


Caledonia .... 


1879 


Columbus .... 


1894 



Location. 



Lead City, 
Lead City, 

Lead City, 

Terravllle, 
Terraville, 

Central City, 

Terraville, 
Central City, 



Number of 
stamps. 



1888. 

80 

120 



120 

80 
80 

100 
80 



} 



1895. 
100) 
160 i 

140 

160 

100 ! 

80 
10 



Owners. 



Mining 



The Homestake Min- 

ingCompany. 
The Highland M 

Company. 
The Deadwood-Terra 

Mining Company 
The Father de Smet 

Mining Company. 
The Caledonia Mining 

Company. 



Before plunging into an account of the milling methods it 
seems needful to apologize for undertaking the task in the 
face of the very excellent description prepared by Prof. H. O. 
Hofman, which appears in Vol. XVII of the Transactions 
of the American Institute of Mining Engineers. My excuse 
is that during the interval of seven years since that article 
was prepared some changes have been made at the mills, and, 
furthermore, I felt that this book would be incomplete with- 
out some reference to the methods of so famous a milling 
center as the Black Hills. To escape unnecessary repetition, 
however, I have avoided dealing with certain aspects of the 
milling practice, which are so fully handled by Prof. Hofman, 
in the paper above referred to, as to render my account of 
them a work of supererogation. 

The immense size of the ore bodies from which the mills 
get their daily supply is well suggested in the accompanying 
photograph of one of the big open cuts. Another illustration 
shows the distribution of the Homestake mills, and the general 
appearance of the Belt in the vicinity of Lead City. 

The ore as it comes up from underground is dumped into 
the rock-breakers. Formerly, until 1889, it went to the mill 
and was unloaded upon grizzlies, from which the coarser por- 
tion went to the crusher floor. The Blake crusher was at that 



PROFITABLE WORKING OF LOW-GRADE ORE. 79 

time in use, but the successful trial of the Gates machine, at 
the Caledonia mill, led to the introduction of the latter all 
over the Belt and to the entire replacement of the former. 
Those in use are as follows : 

Mill. Number of crushers. Size. 

Homestake 8 Gates No. 6. 

Highland 2 Same. 

Deadwood-Terra 2 Same. 

Caledonia 1 Same. 

Columbus 1 Gates No. 1. 

The first reduction of the ore by the breakers at the mine 
instead of at the mill is a decided and very obvious improve- 
ment. It renders more easy the loading of the cars which 
transport the ore to the mill ; it does away with the unequal 
strain upon the mill engine when the rock-breaker, as is too 
often the case, is not worked by a separate source of motive 
power, and it eliminates the larger portion of the dust, which 
is such a nuisance not only to the men at work, but also to the 
machinery of the mill. Furthermore, it enlarges the capacity 
of the ore bins, and does away with the necessity of having 
two series, one for the coarse above the crusher floor, and one 
for the fines below it. 

By having a spare breaker, as is the case at the Homestake 
shaft, where only two out of the three are in use at any given 
time, there need never be any interruption to the regular 
passage of the ore as it comes out of the mine, through the 
crushers, into the mill oars. 

The larger receiving capacity of the Gates and its crushing 
power render it preferable, in large mills certainly, to the 
Blake. The idea of first reducing the ore at the mine is 
common in California, the breaker at the mine being some- 
times followed by a further reduction of the ore by a second 
breaker at the mill itself. This practice is good, because the 
relieving of the stamps of the hard work of first crushing aids 
rapid pulverization in the mortar and gives conditions more 
favorable to successful amalgamation. 

In the accompanying table (No. 2) will be found the chief 
figures telling of the milling practice. The table has not the 
variety which would be evidenced in other districts, because 
the management of the Homestake also controls the neigh- 
boring mills, with a consequent tendency toward a general 
uniformity of methods. 



80 



STAMP MILLING OF GOLD ORES. 



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PROFITABLE WORKINi* OP LOW-GRADE ORE. 81 

Referring to the comparative table, the following notes will ^ 
be pertinent. The total dropping weight of the stamp in allV^ k 
the Homestake mills was originally 850 pounds, but the tap- ' '-'^ 
pets and bosses have been made heavier so as to make the ^^> 
weight now approach 900 pounds. The number of drops per / * 
minute varies, of course. I found it to average nearer 88 than ^^ *^^ 
85, the latter being the figure given to me by the millman. u^ 
The height of the drop appears to have undergone a slight in- 
crease since 1888. At the Deadwood-Terra it was 9 and 9^ 
inches until quite recently. The reason given for the change 
is the idea that a longer interval between the drops permits 
the stamp to turn upon the ore, a purely theoretical consider- 
ation quite secondary to the fact that the slightly increased 
interval is found in practice to diminish breakages by pre- 
venting the horn of the cam from striking the tappet too soon. 

In the matter of the depth of discharge the variation is due j;.^ 
to the wearing down of the dies. The importance of this '^ V' 
factor in gold milling is universally underestimated. At the 
Homestake a serious effort is made to obtain some approach 
to uniformity in the issue by the use of chuck-blocks of dif- 
ferent sizes. The chuck-block serves as a false lip to the 
mortar; it is in the form of a piece of 2-inch plank bolted 
to a l|-inch board, the latter projecting about 2 inches beyond 
the former, to which a copper plate is attached. The replace- 
ment of the 2-inch plank by i-inch iron, a method introduced 
some years ago, has been abandoned since the slight increase 
(If inches) in the distance between the chuck-block and the 
shoe was not needed in a mortar whose chief characteristic is its 
narrowness and whose primary intention is to promote rapid 
crushing. In the old Coledonia there were two inside amal- 
gamating plates because the design of the mortar permitted 
it and the character of the particular ore suggested it. In the 
Homestake mills, however, there is only one inside plate 



K 



f 



which is of plain copper and 5 inches wide. When new dies '^ ^< { 
are introduced they are accompanied with the use of a chuck- 
block 7 inches high, making the distance from the bottom of 
the screen to the top of the die about 9 inches. After two 
weeks have elapsed this 7-inch chuck-block is replaced by one 
only 5 inches high, the difference between the two being 
approximately equivalent to the wearing down of the dies. 




^l 



82 STAMP MILLING OF GOLD ORES. 

The copper on the first chuck-block is straight, on the other it 
is slightly curved and mounted on thicker wood so as to bring 
it nearer the stamp. The aim is to have the bottom of the 
chuck-block level with the bottom of the shoe. After from 
four to six weeks the dies are worn out, and at that time the 
discharge has reached a maximum of about 11 inches. 

At the Deadwood-Terra three chuck-blocks, 5, 6, and 7 
^^k^^ Dj p inches high, respectively, are in use. It is intended to keep the 
^"^iMjpp of the die about level with the bottom of the chuck-block. 
The discharge increases from a minimum of 9 inches to a 
maximum of over 11 inches. In the Columbus mill two inside 
plates are employed. The front one is 5 inches wide, and is 
attached to a 5-inch chuck-block. The back one is 8 inches 
wide, and is fastened to the mortar itself. Although the 
latter has only 12 inches of inside width, both plates do good 
work. The violent agitation of the pulp and the scouring of 
the surface of the plate, which might be expected to occur in 
so narrow a mortar, are minimized by the fact that the issue 
is so deep that the stamp is never lifted clear out of the water, 
but produces a steady pulsation rather than an irregular 
splash against the screen and the inside plates. The Columbus 
mill, however, crushes less ore than those on the other side of 
the hill. The mortars are arranged in pairs, with a passage- 
way between each pair. The automatic feeding of the ore is 
done by Hendy " Challenge " ore-feeders in four of the mills 
(the De Smet, Golden Star, Homestake, and Deadwood- 
Terra), and by Tulloch feeders in two (the Highland and 
Columbus). The relative merits of these two machines are 
now well recognized, the main points of distinction being that 
the Hendy is much the more expensive, but it is also the most 
automatically perfect, and can treat wet ore as well as it can 
dry millstuff, for feeding which the Tulloch is not so well 
adapted. 
r% In the matter of screens the mills under the Homestake 

'- ^Xjulv management are uniform in the use of No. 8 diagonal slot- 
^ punched Russia iron, equivalent to 30- wire mesh. They 
usually break before they become worn out, owing to the 
Russia iron being brittle along the edges of the rows of slots. 
They are not, however, in any case permitted to remain in 



PROFITABLE WORKING OF LOW-QRADB ORE. 83 

ase until worn out, because the openings would be so enlarged 
as to make the crushing too coarse. At the Homestake mills 
they are never used for more than two weeks, but, as a matter 
of fact, they are generally thrown out, owing to breakage, 
after from six to eight days. At the close of their first week 
of service they are turned upside down. The chief cause of 
breakage*is the chips from mine timbers which become mixed 
with the millstuif and impinge against the screen so as to 
dam up the pulp and jxpose the punched iron to a pressure 
beyond its strength. At the Deadwood-Terra the screens last 
an average of nine days, because the material treated is 
mostly surface ore, and in such there are less chips, because 
less timbering is done than in the big underground stopes. 
At the Columbus mill 30-mesh brass wire is employed. Three 
sets of screens are in constant use for each battery, and every 
day one is replaced by another, while the third is undergoing 
a cleaning. The screens are first dried and then rubbed with 
a brush such as is used for chinaware, so as to free the wires 
from adhering material. Formerly No. 6 burr-slot screens 
were used at the Homestake mills. Then No. 7 came into 
use, but now the No. 8 smooth slot is claimed to be the best 
variety. It is stated that wire cloth cannot be used because 
of the wood chips in the ore, and that when it was tried a few 
years ago in the Golden Star mill a screen was burst out every 
forenoon. Aluminum bronze slot screens gave good service, 
but proved too expensive. It has always been the custom to 
take out the screens and replace them with new ones before 
they become actually worn out. To retain them would be 
false economy, because their retention too long in service 
means coarse crushing, consequent upon the erosion of the 
edges of the openings. It is much to be regretted that the 
above-mentioned chips should render wire cloth impracti- 
cable, because with such screens not only is there greater pro- 
portionate area of discharge, but, owing to the fact that the 
wires do not wear so easily, the apertures remain of constant 
size and it is simple breakage only which brings the life of 
the screen to a close. 

Mercury is fed into the battery in quantities proportioned ''^^z* 
to the richness of the ore and regulated by the condition of 
the amalgam on the apron-plates. At the four principal mills 

 Also due to the unusual narrowness of the mortars in use. 



/"x 



V 



84 



STAMP MILLING OF GOLD ORES. 



the rate at which the mercury is fed can be judged by the 
accompanying record, covering the two weeks previous to my 
visit. See Table 3. 

Table No. 3. 

CONSUMPTION OP MERCURY AT THE HOMESTAKE STAMP MILLS. 



Date. 






Deadwood-Terra, 
160 stamps. 


Golden Star, 
160 stamps. 


Highland, 
120 stamps. 


Homestake, 
100 stamps. 




Lbs. 


Oz. 


Lbs. 


Oz. 


Lbs. 


Oz. 


Lbs. 


Oz. 


May 1, 1895 

:: «: :: . 
:: l: " : 

" 7, " . 
" 8, « 
« 9, " 
« 10, " 
« 11, " 
. " 12, " 








11 
10 
10 
12 
*9 
10 
11 
11 
11 
10 
11 
10 


5 
2 

2 

12 
6 

11 
6 
8 

14 


12 


25 
*17 
25 
26 
25 
24 
28 
24 
28 
25 
23 
24 


13 

14 

16 

13 

3 

8 

2 

12 

13 

3 

7 

6 


15 
*12 
15 
10 
10 
16 
17 
19 
19 
18 
18 
17 


4 
9 
1 

10 
6 
8 

8 
8 

12 
8 
4 


♦9 
12 

13 • 
12 
11 
12 
12 
12 
12 
12 
12 
11 


10 

2 



12 

7 

2 

8 

15 

18 

18 

7 

1 


Average per day . 


10 


14 1 24 


3 


16 


5 


12 


2 






It will be observed that though the Golden Star and the 
Deadwood-Terra mills crush approximately the same amount 
of ore, the former uses more than twice as much mercury. 
This fact is explained by the wide difference in the richness 
of the ore, for while that crushed in the batteries of the 
Golden Star averages from $4 to $5 per ton, that which goes 
through the Deadwood-Terra ranges from $1.50 to $2 per ton. 

For the year ending June 1, 1894, the Homestake mill used 
2,084 pounds and the Golden Star mill 3,440 pounds, making a 
total of 5,524 pounds avoirdupois, which at the price obtain- 
ing that year, 42 cents, makes the value of the mercury used 
$2,320.13. During that time 309,210 tons of ore were crushed^ 
so that the consumption was at the rate of about 5 dwts. troy 
or i cent per ton. At the Deadwood-Terra 205 pounds were 
used in February, 1895, in treating 18,483 tons of ore. It is 
estimated that 22 per cent, of the amount of mercury used is 
lost. 

At the Homestake Mining Company's mills the water used 
in the batteries is at the rate of 1^ miner's inches per battery 
of 5 stamps per 24 hours. This is equivalent to 23,414 gallons 
per battery, or a little over 3 gallons per stamp per minute. 
At the Deadwood-Terra it is a little more, at the Columbus a 

^Indicates clean-up days. 



86 STAMP MILLING OF GOLD ORES. 

little less. The quantity of water used is proportioned to the 
quantity of pulp crushed and its specific gravity, the intention 
being to so regulate this factor as to produce a slow, wave-like 
movement over the oi:tside amalgamating tables, and to so 
thin the pulp as to aid the separation of the gold without 
having such an excess of water as would rush the crushed ore 
too rapidly over the amalgamating surface. 

In winter the water occasionally runs low. It is then used 
over again by the intermediation of the two setting dams con- 
structed in the gulch below the mills, the water being pumped 
back when the mud has settled. The mine water is also 
often used to supplement the ordinary supply when otherwise 
insufficient. 



CHAPTER VI. 
Milling in the Black Hills, South Dakota. 

In the last chapter the Homestake mills were passed in 
review. Their arrangement and operation is of great interest 
to those engaged in the exploitation of large deposits of low- 
grade ores, such as are likely to be our main source of the 
precious metal in the coming century. 

In their general arrangement the mills follow a common, 
well-planned design. The batteries are placed in two rows 
upon a flat site. In the Father de Smet, erected in the sum- 
mer of 1878, the 16 batteries are arranged in two rows of eight 
facing each other and discharging toward the center of the 
building. This arrangement, convenient in that it brings all 
the amalgamating apparatus under one view, has nevertheless 
in practice been found less serviceable than that adopted in 
all the other plants. In these the batteries are also in two 
rows, but are placed back to back with ore-bins and feeders 
distributed in the space between. The cars on arrival at the 
mill are emptied so as to send half their contents to either 
side into the rows of ore-bins, the bottoms of which make an 
angle similar to a flat inverted letter V, as is shown in the 
accompanying drawing* of the Highland mill. The Father de 
Smet has the largest bin capacity, obtained, however, at the 
expense of darkening the amalgamating tables. Mr. Bowie, 
who designed the mill, states that the ore-bins have at times 
contained 1,500,000 pounds of ore without injury to the 

structure.! 

At the Highland mill the main shaft runs through the 
center of the building, over the battery sills, and connects 
with the counter-Shaft immediately behind the stamps. At 

• Which I owe to the courtesy of the builders, Eraser A Chalmers. 
•{• Transactions American Institute Mining Engineers, Vol. X., p. 88. 



•»,_' 




I  

Ijdinal and ci^ 

I OF 

HlQHLANJ 

Deadwood, So. f\ 




^.] 



Ink 




mn 







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I ' 






BUILT BV 

FRA8ER A CHALMi 

CHICAOO, ILL. I 




L.J 



e 



r 









t 



-.^'a^i>A.. - •»»..^'*'.A* 






ja. 



MILLING IN THE BLACK HILLS, SOUTH DAKOTA. 89 

the Golden Star the driving shaft is placed on an approximate <J* /? 
level with the cam shaft, so that the battery belts are nearly 4^ ' 



6 



horizontal. In the Homestake the line shaft runs direct from 
the engine through the center of the mill along the battery 
sills, and the battery belts extend each way at about an angle 
of 30 degrees. At the Dead wood -Terra the main shaft on one 
section is a continuation of the crank shaft of the engine, th'e 
other takes its belt from this. The counter-shafts are under 
the feed floor just behind the batteries. 

Of these diverse arrangements that of the Golden Star mill n 
•obtains common preference. It is similar to that generally ^ic^ 
in vogue in California. The arrangement to be seen in the j^**< 

old Homestake mill entails a small first cost, it is simple, and 
it gives an equal pull either way upon the main shaft, but it 
is nevertheless not to be preferred to that of the Golden Star. 
The placing of the counter-shaft immediately under the feed 
floor and behind the batteries is objectionable, because 
this is a part of the mill particularly subject to the dust and 
and dirt surrounding the crushing apparatus; it is inacces- 
sible and inconvenient ; it requires the use of tighteners, and 
it is affected by the vibration due to the falling stamps. The 
Highland mill uses up three belts while the Star is wearing 
out one. The arrangement obtaining in the latter places the 
driving shaft nearly horizontal with the cam shaft, dispensing 
with tighteners ; it renders everything well lighted and easily 
accessible, and by the use of a framework independent of the 
battery sills it minimizes the effects of the vibration due to 
the stamps. £.0 

When first erected the order of the drop in any two adja- ^ »' y 
cent batteries was : ^^L 

1.5-9-7-3 I 2-6-10-8-4 1^ 

The pulp used to wash always one way and tended to accu- 
mulate at one end. Now it is : 

1.7.3-9-5 I 2-8-4-10-6 

At the Deadwood-Terra the order in any single battery is 
1-4-2-5-3, which corresponds to the last above given. The order 
1-6-2-4-3 tends to scour the chuck-block and wear the screen, 
because two stamps drop almost simultaneously, but, on the 
other hand, this last arrangement promotes rapid crushing 
better than any other, and many millmen prefer it. 






/^ . 



-•■^'-Mi.: 



90 STAMP MILLING OF GOLD ORES. 

In the matter of the saving of the gold-bearing sulphides 
remaining in the tailings, the mills of the Belt afford no com- 
mendable example. The sulphides of the Deadwood-Terra 
are too low in value to be worth collection, but at the mills on 
the Lead City side of the hill, shaking tables are in use. They 
are the f amilar, very simple and excellent machines known as 
the Gilpin County bumping tables. The number of them in 
use is, however, absurdly disproportioned to the amount of 
material going over them, so that the six concentrators at the 
Homestake and the eight machines at each of the other two, 
the Highland and Golden Star mills, must be considered only 
in the light of a badly planned experiment. At the Columbus 
there is one concentrator, treating the pulp from 5 stamps, 
fhe ore containing 15 per cent., and the concentrator collecting 
only 6 per cent, because it is overcrowded with pulp. 

During the year ending June 1, 1894, the two Homestake 
*mills produced 915,010 pounds of concentrates, whose assay 
value varied from $5 to $8 per ton. They consist of iron 
pyrite, arsenical pyrite, and pyrrhotite. The ore contains 
from 3 to 5 per cent., but only about 2 per cent, of the sul- 
phides are saved. They are sent by rail to the Deadwood & 
Delaware smelter, just below the town of Deadwood, where 
they are treated at a charge of half their assay value and con- 
verted into an iron matte, very low in copper and rich in gold, 
which goes to the Omaha & Grant Smelting and Refining 
Company, at Omaha, for further treatment. 

Turning to the process of gold extraction in the various 
stamp mills, we find that it is carried on in the mortar itself, 
on apron-plates, by means of traps and is finally partially sup- 
plemented by an inadequate eflFort at concentration. 

The mortar becomes an amalgamating machine by the addi- 
tion of free mercury, as already described. From the mortar 
the pulp is discharged through the screens and drops 6 to 10 
inches upon the apron-plates. This drop serves no purpose 
A splash-board, placed so as to face the discharge, receives 
and breaks the fall of the issuing pulp. This prevents the 
sand from scouring the surface of the apron-plate. 

In the Golden Star and Homestake mills the aprons are 10 
c'i^'^*jj^ feet long by ^ feet wide. In the Highland the length is 2 

feet less. Two aprons deliver their pulp to one tail-plate 



/. 



MILLING IN THE BLACK HILLS, SOUTH DAKOTA. 91 

having a size equal to the apron (the general arrangement is 
well illustrated in the photograph of the interior of the 
Golden Star mill. See page 85). Both apron and tail-plate 
have an inclination of 1^ inches per foot. An attempt has 
been made to work the lower plate with a less slope, but the 
gravity of the pulp prevented the plate from clearing itselL 
The millmen complain that the lower plate is hard to keep in 
order, because of the small amount of amalgam which it car- 
ries on its surface. The remedy would be to clean it less 
often, or to use an electro-plated silver surface. Amalgam is 
the best thing for keeping plates in order, and is superior ta 
all the chemicals in creation. 

All the plates are of plain copper. In the Highland mill 
the launders have their bottoms covered with copper. The 
plate which goes inside the mortar is cleaned up twice per 
month ; so also the tail-plates. The aprons are cleaned and 
dressed every morning. The tail-plate is also dressed each day. 

At the Dead wood-Terra the aprons are 11 feet long by 4 
feet 8 inches wide. The sluice or tail-plate is 8 feet long by 
16 inches wide, forming an absurd apparatus for arresting^ 
gold. At the Columbus the apron is 14 feet long by 4 feet 3 
inches wide. The inclination is 2 inches per foot. 

At all these mills there are traps or other similar devices for 1 
arresting escaping mercury and amalgam. In the Golden '?^, 
Star, for instance, there is a 2-inch riffle or trap at the bottom 
of the apron, and at the head of the tail-plate there are twa 
deep traps with a drop of 18 inches. The first-mentioned 
riffle or shallow well is said to do the best work because the 
pulp which goes over it is flowing smoothly and the conditions 
favor the separation of the amalgam and its arrest. In the 
case of the subsequent drop-traps the pulp is so agitated by 
the greater speed of its passage and the depth of the drop 
that opportunity for settling is less. These traps are cleaned 
up every two weeks, the accumulated pyrites are shoveled 
into buckets, and then passed into a pan which extracts all 
the free amalgam. The residues from the pan are then fed 
into a particular 5-stamp battery, provided with a No. 10 slot 
screen. They are passed through this battery twice and are 
then sent to the smelter, their final assay value being about 
$38 per ton. 



; 1 1 



'Mt(' 



92 STAMP MILLING OF GOLD ORE». 

• 'j ^ The above suggests the Australian practice of mercury wells, 

' ^^^^tvj particularly employed at Clunes, with the obvious difference 

f L^((^4^ that the Homestake traps are not supplied with free mercury. 

It is claimed that if sufficient mercury is fed into the battery 

no free gold should escape, and the mercury in the traps 

would merely serve to thin the amalgam and make it easier 

of escape. The traps catch concentrates and amalgam only. 

At the Homestake mills about one-half of the total amalgam 

^^'-'cltiv, comes from the inside plate. The traps catch about 1 per 

V cent, of the total. At the Highland mill 4 to 6 pounds of 

amalgam are obtained from the traps at each clean-up. At 

the Deadwood-Terra quite 70 per cent, of the total amalgam 

, comes from the inside of the battery. 

^^'^<^-iw ^^ effort is made to maintain the battery water as near 
\U 70° F. as possible. In the winter months the exhaust 

pipe of the engine is turned through the supply tank and 
keeps the water warm. If the temperature is raised too high 
it is found that the mercurv becomes too mobile and runs 
down the plates, leaving the amalgam too hard for good ser- 
vice. Very few chemicals are used. At the Golden Star mill 
only 10 pounds of potassium cyanide are consumed per annum. 
It is found advantageous to wash the plates with a weak solu- 
tion of salt and sulphuric acid, a handful of salt and half a 
teacupful of acid in 2^ gallons of water. 
C{ '.. f)^ / ^^^ concentrating apparatus, such as it is, has been described 

^'"^^Iready. Its inadequacy is recognized and experiments are 
now being made with jigs which promise well. Several years 
ago two blanket houses were in use ; the blanketings were 
worth from $20 to $80 in assay value, and were obtained by 
employing Brussels carpets. 

The best comment to offer on the existing methods is to 
state the fact that a Cornishman a few miles below the mills 
has been successfully treating, by means of two simple piston 
jigs, the tailings carried down in the creek. His success 
has attracted the attention of the Homestake management, 
/^ and induced the commencement of the experiments above 

"^'11^ referred to. 

^.it^ /^JL ^^^^ various castings required in the mills, such as cams, 

[^;^ tappets, dies, and even mortars, are made on the spot at the 
c\ -L^ foundry controlled by the Homestake management. In the 






r\ 



MILLING IN THE BLACK HILLS, SOUTH DAKOTA. 93 

matter of breakages it may be stated that at the Deadwood- 
Terra mill 29 stems were broken in April, the ordinary aver- 
age being 20 per month. At the Golden Star the average is 
40 per month. The cast-iron cams, made locally, do not give 
such good service as steel cams imported from the outside, 
but other business considerations cause the management to 
prefer them. The shoes come from Chicago and cost 2 cents 
per pound, to which the freight of 85 cents per 100 pounds 
must be added. The dies, homemade, are estimated to cost at 
the rate of 2 cents per pound. The latter are made of iron 
slightly softer than that of the shoes, and therefore the con- 
ditions for good service, in so far as relates to the relative 
hardness of the metal in the shoe and die, are obtained. 

Chrome steel shoes did not give as good service as the cast 
iron. The chrome steel dies wore better than the iron, but ^^^l fy 
they were rejected on account of their greater cost, at that 
time cents per pound in Brooklyn, to which freight must be 
added. Shoes made of ferro-aluminum gave good results. 
In this connection it may be mentioned that statements 
regarding the service of shoes and dies made of particular 
mixtures of iron or steel are not* always reliable because, 
first, of the varying quality of succeeding shipments of such 
material, and, secondly, the fact that their service is largely 
dependent upon whether both shoes and dies are made of the 
particular metal or whether the shoe is used with a die of a 
different metal, or vice versa. The chrome shoes at the Home- 
stake wore smoothly and cupped slightly. I may here men- 
tion the observation, made by Mr. A. L. Read, of the Golden 
Star mill, that the slight hollowing of the shoe helps the 
crushing capacity of the mill by tending to hold the pulp 
under the stamp instead of splashing it out. This does not, 
of course, refer to irregular cupping, l^ut simply to an even, 
gentle hollowing out of the face of the shoe. ,^ 

The guides in use are the invention of Edmund Major, ^^-^^^ 
formerly a carpenter in the employ of the company. They '' 
are made of maple and are capable of convenient adjustment C 
so as to counteract the tendency of the stem to work away C ^ " 
from the side on which the cam is placed. One stem can be ^ '• , . 
removed without taking down the entire series of guides 
belonging to any single battery. 



% 
' / 



"D 94 STAUP MILLING OF GOLD ORES. 

^ix^ , Key tappets, with two keys, are in general use. The stamps 

r '' of two neighboring batteries revolve toward one another. I 
noted that it took from five to nine drops to enable the stamp 
to make a complete revolution. The variability in this 
respect was mainly due to the quantity of grease upon the 
surface of the cam. 



The Homestahe 1 



The work of the mill used to be checked, not by the regular 
taking of pulp samples, but by the rough testing of the ore 
coming from the mine by the sampler, a man whose business 



I 

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1 



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^ ^/ 



K 



*<>•:; ».-H ' 



I 






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J: I : 3« ' lA-i 



5 ' -^O?.'! J- .'ATciMOH 3HT 



MILLING IN THE BLACK HILLS, SOUTH DAKOTA. 95 

it was by careful panning to estimate the amount of free gold 
in the millstuff. He made a daily report and by that report 
the mine foreman was guided in the distribution of his stop- 
Ing activity. It has always been the aim of the management 
to maintain a uniform monthly output. It is the practice 
now to take samples of the tailings in the afternoon of each 
day at each mill. They are taken every hour for a period of 
five hours by the use of a dipper placed at the foot of each 
apron-plate. The gold contents are determined by fire assay. 
Nothing more careful or more intelligently systematic is ever 
done with a view to determining the actual extraction accom- 
plished in the mills. 

The annexed plan and section, which I owe to the courtesy 
of the makers, Frazer & Chalmers, Limited, of the mortar in 
use at the Homestake mills, will be of service in indicating 
the conditions under which the inside amalgamation is carried 
on. The pattern was changed about 10 years ago, the mortar 
being made narrower and the issue deeper. The latter was 
also further increased by the use of chuck-blocks. The inside 
width of the mortar at the level of the discharge is 13 inches. 
At the Columbus mill it is 1 inch less. For purposes of com- 
parison it may be mentioned that at the Pearl mill, Bendigo, 
the inside width is 15 inches, at the North Star mill, Grass 
Yalley, 17^ inches, and at the Hidden Treasure mill, Black 
Hawk, it is 24 inches. 

The accompanying reproduction from photograph represents 
the latest design of this type of mortar. The makers inform ^^L^. 
me that a number of changes have been made in its construe- >^rt 

tion. The original Homestake mortar had outline dimensions ' *^ 

as follows : Length of base, 4 feet 8 inches; width of base, 2 
feet 4 inches ; height, 4 feet 11^ inches ; weight of mortar, 
6,500 pounds. The last-constructed mortar weighs 7,300 
pounds, and its length of base is 4 feet 8| inches ; width of 
base, 2 feet 4^ inches ; height, 4 feet 10^ inches. The latter 
is provided with cast-steel false bottoms 2| inches thick ; also 
with cast steel liners seven-eighths inch thick along the sides 
of the mortar, and a similar lining one-half inch thick upon 
the feed opening. It is provided upon the inside with copper 
plates one-fourth inch thick and furnished with chuck-blocks, 
each of which is copper lined. The chuck-blocks are made in ^^ 



K 



^Cu 



£/ 



/I- 



^^^'h 

"^ 



96 



STAMP MILLING OF OOLB ORES. 



^.1 






I . 



:• f-'rl 



a series of three, 6 inches, 8 inches, and 10 inches deep, respec- 
tively. The tendency is to depart from the original weight 
of the stamps, 850 pounds, and to use 900 pounds and more. 

Table No. 4. 

SHOWING THE LABOR EMPLOYED IN THE PRINCIPAL STAMP MILLS. 





Deadwood- 






















Terra. 


Homestake. 


Golden Star. 


Highland. 




160 stamps. 


100 stamps. 


160 stamps. 


120 stamps. 




• 




• 
00 


• 




• 
00 


• 




• 


• 


• 


• 
OQ 




a 


Shift, 
hours 


Wages, 
dollai 


a 


Shift, 
hours 


Wages, 
dollar 


a 

s 


Shift, 
hours 


Wages, 
dollai 


a 
y^ 


Shift, 
hours 


Wages, 
dollar 


Foreman 


A 


17.00 


B 


18.00 


B 


18.00 


B 


18.00 


Millwright 


1 


12 


8.50 


1 


12 


4.25 


1 


12 


4.25 


1 


12 


4.26 


Pipefitter , 






• •  


y2 


C 


8.50 


H 


C 


8.50 




D 




Enginemen 


2 


12 


8.00 


2 


12 


8.50 


2 


12 


3.50 


2 


12 


3.50 


Firemen 


2 


12 


2.50 


2 


12 


8.00 


2 


12E 


8.00 


2 


12 


8.00 


Night Foreman . . ' 








r^ 


F 

10 


5.00 
4.00 


r^ 


F 

10 


5.00 
4.00 


r^ 


F 

10 


5.00 


Head amalgamators 


2 


10 


8.50 


4.00 


Amalgamators . . . 


4 


12 


8.00 


4 


12 


8.50 


4 


12 


3.50 


4 


12 


3.50 


Crushermen ... 


3 


10 


^ 2.50 


2 


10 


. 8.00 


2 


10 


8.00 2 


10 


8.00 


Oilers 


2 


12 


2.50 


2 


12 


3.00 


2 


12 


8.00 


2 


12 


8.00 


Feeders 


3 


12 


2..50 1 2 


12 


8.00 


4 


12 


8.00 


4 


12 


3.00 


Laborers 


1 
20^ 


10 


2.00 


2 


10 


2..50 


4 
233^ 


10 


2.50 


8 

21% 


10 


2.50 


Total 




59.00 




04.82 




75.82 




70.07 



A.— The Dead wood -Terra is on the opposite side of the hill from the Homestake 
and, although under that management, has its own foreman. 

B and £. — One day foreman and one night watchman attend to the three mills, 
the Star, Highland, and Homestake. 

C— The Homestake and Star mills share the services of one mechanic. 

D.— The Highland mill has its own mechanics. 

A new mortar lying outside the Golden Star mill invited 
inspection, resulting in the following comments : The feed- 
hole is 4^ inches wide, and has vertical sides, a common but 
injudicious arrangement. The feed-hole should be smaller at 
the top than lower down, so that pieces of ore would drop 
down into the mortar freely, if they entered at all. (In the 
Father de Smet mortar, designed by Mr. Bowie, the top of 
the feed-hole is so narrowed. ) The bottom of the feed-hole 
is too thin. This part of the mortar is the first to wear 
through. The lip is the next part to become worn out. Then 
holes begin to appear in the ends. The Homestake mortar 
lasts from four to five years. The back projection, designed 
to throw the splash to the front of the battery, is too near the 
shoe. The distance between the shoe and this portion of the 
mortar should at least be equal in size to that of the pieces of 



MILLINa or THE BLACK HILLS, SOUTH DAKOTA. 97 

ore which are permitted to enter. As it it less than this, 
pieces of ore often get wedged in and require dislodgement 
with a crowbar. 

An incident worthy of notice was the trial, from November, 
1888, to the middle of June, 1889, of a steam stamp, similar to 
those so successfully used at the Lake Superior copper mines. 
Its size was 11 by 26 inches, and it gave the best crushing 
results with 22-inch strokes and 95 of them per minute. The 
water consumption was 7i miner's inches per 24 hours, when 
crushing from 125 to 135 tons of ore, this being equal, there- 
fore, to about 900 gallons per ton of ore. The steam pressure, 
when doing such work, was maintained at 85 pounds, but with 
110 pounds pressure a maximum crushing capacity of 192 tons 
was attained. 

No. 7 needle-slot screens, made of steel of No. 17 gauge, Q 
were employed. The use of a finer mesh entailed an enormous ' U/: ^ 
waste, owing to the pulp becoming banked up inside, always ^' 

followed by the bursting of the screen. The screens were 
replaced every six days. 

The amalgamating tables were 12 feet long by 4 feet wide, fj 
and the traps were 30 inches deep by 20 inches square inside. ^^^ 
The tables had a fall of 1} inches per foot. Owing to the ^M^ 

rapidity of the crushing no plates could be employed inside 
the mortar, and it was found that even when using screens of 
an identical kind the pulp issuing from the steam stamp was 
coarser than that discharged by the ordinary stamp mill, 
because the issue with the former was more forceful than 
with the latter. The crushing capacity of the steam stamp, as 
measured per cord of wood consumed for fuel, equaled the 
ordinary Homestake stamp, but the former crushed more 
coarsely. Moreover, the jar was such as to cause the amal- 
gam on the tables to ball up, the mercury exuding and running 
away. This is a defect to be remedied by a slight change in 
the construction of the mill. In general it may be said that 
the steam stamp crushed too fast for either concomitant or 
subsequent successful amalgamation. 

Before venturing on a criticism of the milling practice it 
will be necessary to indicate the conditions which have deter- 
mined its character. The mines contain bodies of gold- 
bearing ore which, while low grade, are of such enormous 



a 



98 STAMP MILLING OF GOLD ORES. 

dimensions as to guarantee a large output for long periods of 
time, and justify the big scale on which operations are con- 
ducted. The ore itself is largely chloritic schist, unequally 
traversed by quartz. Certain portions of the country rock 
are quartzose and^carry gold; they are therefore quarried 
above ground and stoped underground. There is no marked 
definition between what is valuable and therefore "ore,"^ 
and what is valueless and therefore "country rock." The 
pay ore usually carries sulphides, of which iron pyrite, 
pyrrhotite, and arsenical pyrite are most noteworthy, and of 
which the last is more particularly the comrade of the gold^ 
The ore is broken without any attempt at sorting. Its low 
tenor and the mode of its occurrence alike render such a step 
impracticable. At one time the porphyry, flanking and over- 
topping the ore bodies, was sent to the mills. It is now 
dropped down to the lower workings and used as filling to 
sustain the timber work. 

In the 12 months ending June 1, 1894, the two Homestake 
mills treated 309,210 tons, and produced $1,390,610, equivalent 
, to a yield of $4.49 per ton of ore. What the ore actually con- 
y^ tained and what the tailings really carried away no one 
r^tk&x>\.\K knows, but there is reason to believe that the loss in the tail- 
ings averages more than $1 and that the average extraction is 
not more than 75 per cent. At the Deadwood-Terra the out- 
put averages about 20,000 tons per month and the product of 
the mill has a value of about $32,000, making the yield per 
ton about $1.60. Even this material leaves a small margin of 
profit over all mining and milling expenses. The tailings are 
said to carry 30 to 45 cents per ton, from which, judging this 
to be the minimum, I would estimate the extraction to be 
slightly under 75 per cent. 

The ore is indeed rushed through, but that is because it is so 
low in gold contents that slower treatment would cost too 
much per ton. It is reduced by methods both ancient and 
simple, but that is because the millstuff itself is of simple 
character, and neither invites nor warrants the use of com- 
plicated processes. 

It is the old question of pushing technical perfection of 
treatment until it prejudices commercial success. Not that 
the two are antagonistic, except when one of them is carried 



MILLING IN THE BLACK HILLS, SOUTH DAKOTA. 99 

further than common sense suggests. Thus, there is no doubt 
but that if the mills were set to crush the ore at the more 
ordinary rate of 2 tons, per stamp, the extraction would be 
increased, say 10 per cent, equivalent to a gain of 45 cents per 
ton, but then the cost of milling would be, if not doubled, at 
least largely augmented, equivalent to an increase, say, of 60 
cents per ton. By crushing slower and thinning the pulp 
passing over the plates, amalgamation would be improved, 
but the increased extraction would be far more than offset by 
the added cost. Nor, it must be remembered, would this 
rapidity of treatment be accompanied by so small a loss, com- 
paratively, were it not for the fact that the ore is of great 
simplicity, and yields up its gold readily to simple methods. 
In this respect it even surpasses the free-milling ores of the 
main belt of the foothills of the Sierra Nevada in California. 
Such results, having in view the character of the ore and 
the rapidity with which it is rushed through the mills, is, I 
consider, very good work. It has been said by unfriendly 
critics that "haste and waste" best describes the methods of 
the Homestake Company. It has been urged that the methods 
are crude and extravagant in their seeming want of proper 
appliances to make the gold extraction complete. Such com- 
ment is largely due to a misconception of the objects aimed 
at. The milling practice from a purely technical standpoint 
is insufficient and inadequate, but from the commercial aspect 
of the matter it commends itself on the whole, barring certain 
details, as eminently successful. 

The successful results are largely due to the pattern of the 
mortar. The Homestake mortar, in my opinion, is excel- 
lently designed to give the two results usually desired by the 
millman, namely, a large degree of amalgamation inside the 
battery, accompanied by a minimum interference with the 
rapidity of the crushing. 

Before concluding it may be permissible to point out some 
respects wherein the milling appears open to criticism. The 
collection of the sulphides remaining in the pulp after amal- 
gamation is, of course, totally inadequate, but it will be sub- 
mitted that the present method is only experimental, an ex- 
cuse which does not explain why so many years have been per- 
mitted to elapse without a proper settlement of the problem. 



, J 
J > 



100 



STAMP MILLING OF GOLD ORES. 



O 



a^c U 



C«- 



Two aprons deliver their pulp to one tail-plate of a size 
equal to a single apron. The gradient is the same in both 
cases. Therefore the lower plate is called upon to treat twice 
the quantity of pulp which goes over a single apron, and it is 
expected to do efficient work despite the well-understood fact 
that whatever gold escapes the apron must be finer, or other- 
wise more difficult to save, than that which has been arrested 
upon it. Instead of two aprons delivering to one tail-plate, 
one apron ought to pass its pulp on to two tail-plates, and by 
using additional water the pulp would be so spread over an 
increased amalgamating surface as to add largely to the 
chance of successful extraction. Such at least should be the 
idea instead of the reverse. 



Table No. 6. 

THE COST OF STAMP MILLING IN THE BLACK HILLS, 

SOUTH DAKOTA. 



The HoTTiestake Mill. 


80 stamps. 
1887-1888. 


80 stamps. 
188&-1889. 


80 stamps. 
1893-1894. 


Tons treated 


fl6.7»0 
10.^1 
0.0180 
0.1729 
0.2766 


106,780 

10.2396 

0.0046 

0.1562 

0.2230 


104.996 


Labor 


10.2543 


Supplies 


0.0106 


Water 


0.1986 


Wood 

Coal 


0.0697 
0.1784 


Machinerv 


0.0922 
0.0009 
0.0016 
0.0108 
0.0070 


0.0893 
0.0084 
0.0014 
0.0063 
0.0139 


0.1097 


macninery . . . . . . . . •••••; • • • 


0.0084 


Candles 

Quicksilver 


* 6.0088 ' 


Lumber 

Timber 


0.0167 
0.0156 










Total cost per ton of ore 


10.8406 


10.7415 


$0.8551 



The Qolden Star Mill. 



Tons treated 
Labor . . . . 
Supplies . . . 
Water . . . . 

Wood 

Coal 

Machinery. . 

Oil 

Candles . . . 
Quicksilver . 
Lumber . . . 



Total cost per ton of ore 



120 stamps. 
1887-1888. 



146,566 

$0.2138 

0.0079 

0.1712 

0.2739 



0.1220 
0.0084 
0.0014 
0.0252 
0.0054 



120 stamps. 
1888-1889. 



161,766 

10.1756 

0.0044 

0.1622 

0.1969 



0.1088 
0.0066 
0.0014 
0.0082 
0.0088 



$0.8292 



$0.6718 



160 stamps. 
1898-1894. 



204,216 
$0.1656 
0.0121 
0.2048 
0.0846 
0.1637 
0.1067 
0.0021 



0.0071 
0.0160 



$0.7012 



The wide tail-plates have only been in use three years. 
They were formerly 20 inches wide. At the Deadwood-Terra 
they are 16 inches wide and form a truly idiotic amalgamat- 



• •. 



« f 



MILLING IN THE BLACK HILLS, SOUTH DAKOTA. 101 

ing device. "Sluices" they are aptly termed, since they are 
excellently devised for carrying off the pulp, the mercury, the 
gold, and everything else. 

All the plates at the Homestake are of plain copper. Ex- 
perience elsewhere commends the electro-plated copper, and 
especially in large establishments which can afford to do their 
own plating. The use of a 2-ounce or 4-ounce plated copper 
is especially to be recommended in the case of the tail-plates 
of the Homestake, since this would render it easier for the 
millman to keep them in order and would, I believe, increase 
their efficiency. 

Thus, in spite of the incompleteness due to a want of a pro- 
gressive spirit in the management, the mills of the Home- 
stake offer an excellent illustration of commercial success in 
the working of unusually large deposits of low-grade gold- 
bearing ores. 




V \, 



CHAPTER VII. 
Early Australian Methods. 

Clunes is famous in the history of the Colonies as the local- 
ity where on June 29, 1851, J. W. Esmond discovered the first 
gold in Victoria.* Its importance as a mining center has 
never been equal to that of the neighboring towns of Ballarat 
and Bendigo, but it is probable that no Australian mining 
district has done more useful work for the advancement of 
milling and mining. The history of its premier mine — the 
Port Phillip & Colonial — forms a large part of the early 
record of colonial "quartz reefing," and it is certain that in 
the history of milling in Australia that of the " Old Port 
Phillip " batteries forms the most important chapter. 

Clunes commenced quartz mining in the Colonies. While 
Ballarat was astonishing the world with the rapidly succeed- 
ing discoveries of nuggets of wonderful size, and while Ben- 
digo, still exploiting the rich alluvium, had not yet learned 
the value of the lodes whose white croppings were then only 
natural curiosities, Clunes was quietly laying the foundations 
of a great industry. It was fortunate that the difficult work 
of beginning was in the hands of the men who directed the 
affairs of the old Port Phillip. The Port Phillip & Colonial 
Gold Mining Company commenced operations in 1857, at a 
time when the opinion was generally held, owing to the rash 
generalizations of Sir Roderick Murchison, that the gold in 
quartz veins was confined to a comparatively shallow horizon. 
From 1857 to 1881, from surface to 1,400 feet, the mine pro- 
duced 1,204,908 tons of quartz, yielding gold to the value of 
£1,946,989, or at the rate of 7 dwts. 14 grs. per ton. The 
dividends which were paid amounted to £481,455. f 

*It was in August of the same year that gold was found at Bunnlnyong, starting 
the stampede to Ballarat, and in November that the Bendigo " rush " broke out. 

f For these and other figures I am indebted to the courtesy of Mr. B. H. Bland, 
the manager and director of the company. I am also indebted to an interesting 
account of the mine by him, entitled, "The History of the Port Phillip <& Colonial 
Gold Mining Company.'' 

(102) 



EARLY AUSTRALIAN METHODS. 103 

The portion of the mill which was first erected commenced 
crushing in May, 1857. At that date the treatment of gold 
quartz was a problem completely unsolved, and in the early 
years of its history the Port Phillip mill laid down the basis 
of Colonial milling practice. In 1861 assays proved the loss 
in the tailings to amount to 6 dwts. 1 gr. per ton. By numer- 
ous changes suggested by careful experiments, this loss was 
decreased until in 1870 it had been diminished to 17 grains. 
In 1862 the collection and treatment of the pyrites was com- 
menced. In 1864 the plant was increased to 80 heads, and 
the first buddies were placed in position. In 1865 the first 
rock-breaker was introduced.* 

As indicating the character of the work done at an early 
date, the quotation of the following figures is permissible : 



The ore from the mine passes through two rock-breakers, 
preceded by sizing-bars ("grizzlies"), before entering the 
mil], which consists of several sections erected at different 
periods. 



Number of beads. Weight □( h< 



There are four stamps to each mortar box ; four sections, 
three batteries each, on one side of the building, and three 
sections, two of three and one of two batteries, upon the 
other. The stamp heads or shoes are square. The mortars 



 PreTlouB to that time the ok 
and oruahed. This practice has i 
Victoria and New Sooth Wales. 



104 



STAMP MILLma OF GOLD ORES. 



are provided with back and front discharge. The crushing 
capacity is at the rate of 2 tons 12 hundredweights for the 
56 light stamps, and 3 tons 12 hundredweights for the heavier 
section. The speed is at the rate of 82 drops per minute, and 
the drop has a height of 8 inches. The issue or depth of dis- 
charge is maintained as far as possible at 4^ inches. The 
grating is of copper, pierced with 81 round holes per square 
inch. The pyrites concentrated (on Munday's Cornish 
buddies) has amounted to three-fourths per cent, of the ore 
crushed. Its average contents have been 4 ozs. 1 dwt. 14 grs. 
of gold per ton. The bullion is of 23.1.5 carats, or 965 fine. 
The retort percentage has averaged 38. 

The business of the mill has always been carried on in a 
most systematic manner. The following tabulated statement 
of product is taken direct from the mill records for the four 
weeks ending May 21, 1873 : 



Where amalgam was 
produced. 



Beds or mortar boxes 

Boxes or wells 

Blankets 

Mills 



Total 







Retorted. 


Ozs. 


Dwts. 


Ozs. 


Dwts. 


1466 
708 
406 
883 


2 


678 

249 

121 

96 


11 

12 

12 

7 


2966 


5 


1141 


42 



Per cent. 

of 

total. 



60.02 
21.87 
10.66 

8.46 

100.00 



The other statistics were as follows: Number of stamps, 
80; tons crushed, 6,023; hours worked, 618, or 21.68 days; 
average duty per stamp, 2.9 tons ; yield per ton, 4 dwts. 10.12 
grs. ; loss in tailings per ton, 20.16 grs. total contents per ton, 
5 dwts. 6.28 grs. 

Of the total quantity crushed, 2,702 tons, or more than half, 
passed through the rock-breakers. Of the amalgam, that 
coming from the " beds " or mortar boxes retorted 46 per cent. ; 
from the " boxes " or wells, 36 per cent. ; from the blankets, 
30 per cent. ; from the Chilian mills, 26 per cent. 

Of the total product obtained by direct amalgamation, more 
than half came from the mortar box, indicating the free- 
milling nature of the ore. No mercury was used in the 
mortar box. Of the total, 80 per cent, went no further than 
the wells immediately outside the mortar box. 

The lower part of the sheet indicates the character of the 
extraction during that particular month. Of the average 



EARLY AUSTRALIAN METHODS. 



105 



contents of the ore — viz, 5 dwts. 6.28 grs. — only 20.16 grains 
were lost, giving a yield of 4 dwts. 10. 12 grs. per ton, equal to 
84 per cent, of the contents of the ore. At the present time 
the Port Phillip batteries are idle, but the milling practice 
which they inaugurated is to be seen reproduced in a modi- 
fied form in the newer mills of the South Clunes United 
and the Dixon's North Clunes. The comparative table will 
illustrate the different features of the methods employed : 



COMPARATIVE TABLE OF THREE 


CLUNES 


MILLS. 












• 


• 




i 


• 

B 






• 




1 
d 






u 
Qi 


« 








a 


-4^ 

a 


• 


03 


08 

QQ 






a 




o 

d 


OQ 

0) 




g 


d 


a 






08 

QQ 


•P4 

a 

u 


2 

•0 

o 


•r-l 
V4 


o 

Pi 


• 


.• 


a 

DQ 

0) 


09 

g 





d 
a> 

d 




2 

a 


49 

bo 


ersta 
e. 


MiU. 


• 


49 

ft 


Pi 
OD 


ft 


o 

XI 


i 




bo 

a 

4^ 


p. 

OS 

o 


a 

0) 

o 


d 0) 

if 08 


d 
o 

•V4 


• 

»4 

o 


o . 
nd 


o 

OS 


inut 




1 

02 


0) 


2 


W 

CO 

,a 


P4 

OQ 

xi 


08 

• 

OQ 


• 

d 


08 

o 

• 

•1^ 


»-4 

o 

w 


d 
o 
u 

• 

o 

Wi 


d u 


• 


4^ 

• 

o 


OQ O 

• 

QQ 


0) 

• 
OQ 






• 

S8 


m 

OD 


• 

QQ 






Xi 




d 


fl 





o 


OS 




« 


a>»-i 


a> 


»H 


o8 


08 




(56 
24 


>A 


82 


M 

8 


M 


Eh 
8 


H 
240 


u 


81 


0^ 


o <^ 


04 

970 


88 


o 


p 

80 


O 


Port Philip . 


728 > 
896 


4 1 


6 


S.CPn United 


60 


896 


80 


8 


4H 


2H 


150 


0« 
o 


100 


% 


8 5 


968 


42 


51^ 


25 


8 


Dixon's N. 
















o 
















Clunes . . . 


80 


896 


80 


8 


7 


8^ 


100 




180 


8 


8 


978 


40 5^ 


« 


10 



The South Clunes United mill contains 60 stamps in six 
sections of two batteries of five each. The weight of the 
stamp is 8 hundredweights. The speed is at the rate of 80 
drops per minute. The height of the drop varies from 6 to 8 
inches. The depth of discharge or issue is kept fairly constant 
at 4^ inches. As the die wears down, sand is packed under- 
neath, and when about 2 inches have been worn away, a 
second "false bottom" is placed under what remains of the 
die. This false bottom consists of a plain iron casting of a 
sufficient length to serve for two dies. One of half the length 
is used for the center stamp. The rate of crushing averages 
2.4 long tons per 24 hours. In 12 months, working 16 hours per 
day and 6 days per week, there were crushed 28,820 tons. 
The " grating " or screen is of copper plate, 1^ pounds of 
copper per square foot. It is perforated with 100 holes per 
square inch. The average wear at present is about a month, 
or, say, 25 working days, working full time. Iron-punched 

* Information not obtained 



106 STAMP MILLING OP GOLD ORBS. 

gratings were found not to last for a week. The percentage 
of concentrates is usually 1 per cent., having increased slightly 
as depth has been attained in the mine. In the upper work- 
ings it was three-fourths per cent. The concentrates usually 
carry 3 ounces of gold per ton. Just now, however, the ore is 
poor and is yielding at the rate of 178 tons 19 cwts. 3 grs. of 
pyrites, worth £660 12s. 18d., from the crushing of 28,820 
tons of ore. The bullion is 23^ carats, or 969 thousandths fine. 
The percentage of gold in the amalgam varies from 86 to 45. 
The water consumed is at the rate of 8 gallons per stamp head 
per minute. The loss of mercury is unusually small and 
amounts to 5^ grains per ton of ore crushed. 

We will now follow the ore through the different stages of 
its treatment. The millstuff is trammed from the mine and 
discharged into the ore-bins. There are no rock-breakers, 
but self-feeders of a simple pattern pass the ore on into the 
mortar boxes, which are of peculiar design and are provided 
with both back and front discharge. No mercury is used in 
the mortar box. The pulp issuing from the battery passes 
through wells and then over blankets. The blanket washings 
are treated in revolving barrels with the addition of mercury. 
From the blankets the tailings go to Cornish buddies, which 
are further supplemented by tyes placed outside the mill. 

Such, briefly, is the mode of treatment. There are many 
interesting details to be noted. The mine is distant one-third 
of a mile from the' mill, and the tramming and breaking of 
the ore are done under contract for 8 pence per ton. The 
feeding of the ore is regulated by a simple contrivance which 
is shown in the accompanying Fig. 1.* H indicates the lower 
end of the shoot leading from the ore-bin ; to it is attached 
the lower end G of the iron rod i^ which, at its top end, has 
a disc E. This disc ^is keyed to the rod and projects under 
the false or extra tappet D upon the stem of the center stamp 
of a battery of five heads. When the feeding is low the 
stamp falls further than usual, and in so doing causes D to 
strike E^ which communicates the shock to the ore-chute H^ 
and so causes the ore to fall forward into the feeding hole N 
of the battery. 

*This drawing I owe to Mr. Thomas Hewitson, the manager. I am also under 
obligations to him for information which he gave me. 



The pulp pMsei over 
ISfeetof bl&aketi, 

then through bud- 
dies and afterward 
over 2S feet or toll- 
IngH, blankets and 

Flg.l.-Bectl0D of the Stamp Mill of the South dunes United Co., CluneB.VIctorU 



108 STAMP MILLING OF GOLD ORBS. 

The disc or tappet is kept in place by keys. The order in 
which the stamps drop is 8, 5, 2, 4, 1. The shoes are of cast 
iron, 10 inches in diameter and 10 inches high. The dies or 
false bottoms are hexagonal in section ; they are made of 
wrought iron and have a diameter of 10 inches and a depth 
of 6 inches. New shoes weigh 196 pounds; new dies 140 
pounds. A shoe will crush 90 tons, and a die 420 tons, before 
it is worn out. Cast-iron shoes cost 12s. 6d. per hundred- 
weight, and wrought-iron dies lis. 6d. per hundredweight, 
delivered at the mill. 

The mortar box is provided with a double discharge, behind 
as well as in front. In both cases the distance from the bot- 
tom of the screen to the top of the die is kept at about 4^ 
inches. The screens or gratings are similar in coarseness. 

The accompanying drawing illustrates the mortar and its 
accessory appliances. The front grating frame P is 5 feet 
long by 13 inches wide, while that behind, 0, is 5 feet by 12^ 
inches. Both are placed in a vertical position and are covered 
by a " splash board," which slopes forward. The pulp issu- 
ing at the back passes over the lip Q and is conducted by the 
launder U to the front of the battery, where it unites with 
that which is being discharged in front. 

The pulp discharged through the front grating passes over 
the lip "F, and, uniting with that from the back of the mortar, 
goes over a perforated iron plate ^, called the distributor^ 
by passing over which it is evenly spread over the width of 
wells and blankets which follow. This plate is 1 foot deep 
and 3-16 inch thick, perforated with holes 5-16 inch in 
diameter, drilled at the four corners of a square inch. Then 
follows the " apron" S^ a plain wooden table 20 inches deep 
and 2 inches thick, which further aids the even distribution 
of the pulp. Two wells, Tand W, succeed. They are covered 
and guarded from theft by a wooden rack kept under padlock. 
The first well, which succeeds the apron, has a drop of 10' 
inches and a depth of 4 inches. It holds 50 pounds of mer- 
cury. It will be noted that the upright board X compels the 
pulp to pass through the body of quicksilver in the trough 
before escaping, and so insures a contact with the quicksilver. 
The second well, TT, which follows immediately after, has a 
drop of 8 inches, a depth of 4 inches, and also contains 50 



EARLY AUSTRALIAN METHODS. 109 

pounds of mercury. These wells, including the lip, are of 
cast iron ; they have a curved inside contour, and are sunk 
into the wood of the frame which holds them. It has been 
found that iron wells are preferable to wood, since the iron 
has a beneficial effect upon the mercury, tending to keep it 
*' lively," or chemically active, much as in pan amalgamation. 
The wells have an inside diameter of 3 inches, and are placed 
so as to have a slight slope to one end, where a tap-hole 
renders easy the removal of the mercury at cleaning-up time. 
The pulp now goes over the blanket tables Y. The blankets 
are spread upon tables having a width which takes in both 
of the two 5-head batteries forming a section. The total 
width is subdivided into seven partitions, each 18 inches 
across and 12 feet long. The grade is three-fourths inch per 
foot. Then follow five improved circular Cornish buddies 
(Munday's patent), and finally the tailings pass over the 
" tyes," which are outside the building. These last have a 
length of 20 feet and a fall of 1 inch per foot. 

The gold saving is done by the mortar box itself, by the 
mercury troughs or wells, and indirectly by the blankets, 
buddies, and tyes. In the mortar box or coffer no mercury is 
employed. The use of copper gratings would in itself prevent 
it, but the very free character of the gold does not necessi- 
tate its use at this stage of the treatment. The mortar box 
is a roomy one, and gives the gold an opportunity to separate 
itself from the pulp by the action of gravity alone. The 
dimensions are : Interior length of mortar, 68 inches ; interior 
width, 16 inches; distance between dies, 1 inch; distance 
from end die to side of mortar, 2 inches ; distance from die to 
back of mortar, 4 inches; distance from screen to die, 3 
inches ; distance from center to center of dies, 11 inches. The 
mortar boxes are approximately rectangular in horizontal 
section. 

In cleaning up, the grating frames are taken down and the 
material found inside, between, and around the dies is 
shoveled into buckets and then passed over a common strong 
wire sieve or ^ riddle " 2 feet in diameter and of No. 4 mesh. 
One of these lasts for 12 months. The roughs from this oper- 
ation are returned to the mortar box, and are used to reset 
the dies before starting again. At each fortnightly clean-up 



110 STAMP MILLING OF GOLD ORES. 

from li to 2 small bucketfuls are obtained. The fine is sifted 
into a blanket trough and then introduced into an amalga- 
mating barrel. 

There are five such barrels in the mill. The accompanying 
sketch, Fig. 2, will illustrate the arrangement. A is the barrel 
itself, having a capacity of 54 gallons. It makes 16 revolu- 
tions per minute, and is worked from 8 to 12 hours, 10 hours 
being the usual period. The water used is not warm. Seventy- 
five pounds (a bottle) of mercury are added to ea«h charge, 
together with a bucketful of wood ashee. 



Fig. 2. 

When the amalgamation is completed the contents of the 
barrel are emptied into the wooden tank or box S, to be dis- 
charged first over a perforated iron plate C, and then to pass 
on to the three drop wells D, E, and F^ having a drop of 12, 
9, and 6 inches respectively. Nearly all the amalgam is 
caught in the top well, a small proportion only reaches the 
second, while the third is merely a safeguard and is only 
cleaned up occasionally. This disposes of the treatment of 
the residues found in the mortar box. 

We next come to the wells, which are cleaned up once per 
week. The mercury is squeezed through canvas cloth and the 
excess of quicksilver separated from the amalgam. The skim- 
mings taken from time to time from off the surface of the mer- 
cury in the wells, and consisting largely of heavy pyrites, are 
treated in Eerdans,* of which there are three, each 8 feet in 
diameter. The blankets are washed in tubs, the first row 

Thich the grinding is done by & ball 



EARLY AUSTRALIAN METHODS. 



Ill 



every hour, the second every alternate hour, and the third 
every third hour. With rich ore the washing is done more 
frequently. The blanketings or residues from the washing of 
the blankets are treated in the amalgamating barrels in much 
the same manner as described in the case of the mortar-box 
residues. The material collected by the "tyes," straight 
troughs in which heavy pyrites, escaping amalgam, etc., 
collect by the action of gravity alone, is also treated in a 
barrel. The tailings from all the barrels go to certain of the 
buddies. The pyrites obtained from concentration on the 
buddies is roasted in a reverberatory, and then ground in a 
Chilian mill, with the addition of mercury, which collects the 
gold in the form of amalgam. 

The total yield of the mill is variously distributed. In one 
month 2,973 tons were crushed, yielding 981 ozs. 19 dwts. 12 
grs. of gold. Of this total the different parts of the mill 
contributed as follows : 



Mortars (beds) 

Wells (boxes) 

Blankets (by the barrels) . . . 
Skimmings (by the Berdans) . 
Tailings (by tyes and barrels) 
Concentrates (pyrites) 17 tons 



Amalgam. 


Bar gold. 


Ozs. 


Dwts. 


Ozs. 


Dwts. 


955 


5 


) 




644 


19 


} 840 


19 


364 


15 


) 




167 


14 


58 


2 


24 


10 


7 


5 


810 


4 


80 


18 



Retort 
per cent. 

Per cent. 

86 to 48 

82 
80 
26 



It will thus be seen that of the total amalgam saved in the 
mill itself (neglecting concentrates and tailings) the per- 
centage is thus distributed: Mortar boxes, 44.8 per cent.; 
wells, 30.2 per cent. ; blankets, 17.1 per cent. ; skimmings, 
7.9 per cent. It must be remembered that in the case of the 
yield from the mortar box the gold is actually saved as such, 
and it is only in the after-treatment that mercury is used to 
collect it. 

The loss of mercury for the past seven years has averaged 
5| grains per ton of ore crushed. Occasionally the loss has 
risen up to IJ ounces per ton of ore, this being due to copper 
which formed copper amalgam. Such copper amalgam floats 
upon the surface of a bath of mercury and is readily carried 
away with the tailings. This was not due to the abrasion of 
the copper gratings, as might be imagined, but was owing to 



112 STAMP MILLING OF GOLD ORES. 

the presence in the ore of particles of native copper. At one 
time as much as 80 ounces of copper were collected in one 
month from the skimmings of the wells. The wearing of the 
copper of the gratings does no appreciable harm. 

The total consumption of mercury, including the treatment 
of pyrites by Chilian mills, as well as in the mill proper, during 
11 years— from July 1, 1881, to June 30, 1892— has been 3,302 
pounds. During that time 309,400 tons of ore were treated, 
so that the loss amounts to about 3 pennyweights per ton of 
ore. The loss at the Port Phillip & Colonial mill during 
seven years was, including treatment of pyrites, about 5^ 
pennyweights. A good deal of this loss was, however, subse- 
quently recovered. 

The gratings or screens used invite examination. Mr. 
Hewitson, the manager, informed me that up to the year 1879 
the gratings used were imported from England. They were 
made of copper plate, one-sixteenth of an inch thick, and 
drilled with 81 holes per square inch. When in full work the 
imported article lasted 12 months, or during the passage 
through the grating of 2,200 long tons of ore.* At the Port 
Phillip, owing to the smaller area of discharge, the life of a 
grating reached one and a half years. 

The protective tariff caused the imported grating to become 
too expensive, and the domestic article took it place. This 
wore for less than half the time of the imported one. As 
compared to the ordinary punched iron it lasted, however, six 
times as long. The experience with the present lighter type 
of copper grating has been very good. During the past seven 
years 258 gratings have been used up ; their cost was £197 
6s., and during that period 181,792 tons were crushed, or at 
the rate of 355 tons (397 short tons) during the life of a grat- 
ing. It was found that the ordinary round-punched Russia 
iron lasted scarce a week as against a month for this type of 
copper grating. 

Baize is used for the blanket strakes. For one year the 
cost under this head amounted to £47 9s. 2d. During the 
same period the wages at the mill amounted to £1,306 4s. 9d. 
for the treatment of 28,820 tons of ore, or lO^d. (21 cents) per 
ton. The total cost of milling, including supplies, wear and 

* That is, the front and back gratings together are passed through 4,400 tons. 



EARLY AUSTRALIAN METHODS. 113 

tear, treatment of pyrites, etc., amounted to 2s. 3d. (54 cents) 
per ton of ore. 

The Dixon's North Clunes mill is very similar to that just 
described. A few minor differences may be noted. The front 
grating is of copper, 180 holes per square inch, while the back 
grating is of brass wire, 230 to 240 holes per square inch. In 
this mill there are six of Munday's patent buddies with iron 
scrapers, two to each 10 stamps. At the South Clunes United 
there are only five to the 60 stamps ; not enough. Assays are 
made daily, and the tailings are found to be very clean. The 
pyrites is washed and then treated in a Chilian mill. Cost per 
ton, £1 16s. lid. The roasting of 85 tons 1 hundredweight 
cost £89 6s. 7d. ; grinding, £67 15s. lid., or a total cost of £157 
2s. 6d. The furnace is a reverberatory, 40 feet by 5 feet. 

Returning to the examination of the figures given in the 
comparative table it will be noted that both the newer mills 
follow very closely upon the lines of the old Port Phillip. The 
weight of the stamps — 8 hundredweights — is that which is most 
usual in the Colonies. Of the three mills the South Clunes 
United is the only one without a rock-breaker, and the result 
is to be seen in the smaller crushing power. The Dixon's 
North Clunes uses the finest grating, but in so far as this 
affects the rate of crushing it is fully compensated for by a 
less depth of discharge. The weight of the drop is practically 
the same in the three mills. Although the gold in the Dixon's 
is probably coarser than that in the ore treated at the South 
Clunes, the retort percentage is not quite so high because the 
finer size of grating used at the Dixon's produces finer crush- 
ing, and causes the amalgam to be somewhat more contami- 
nated by pyrites. The large quantity of water consumed at 
all these mills is necessitated by the double discharge and the 
use of very wide blanket tables. The consumption of mercury 
is extremely low. The chief source of loss in a mill, that due 
to the flouring of the mercury by its violent agitation with 
the particles of the pulp when under the stamps, is here 
avoided, since none is put into the mortar box ; 5^ grains per 
ton is probably the smallest loss of mercury in a gold mill of 
which we have record.* 

* The other record, that for extreme waste, is held by the Caledonia mill, at the 
Thames, N. Z., where it is stated that one ton of mercury was used up in two weeks 
by a mill of 20 heads only. 



V 



114 



STAMP MILLING OF GOLD ORES. 



Before venturing to criticise the methods of gold extraction 
employed at the mills of this district, it will be necessary to 
consider the character of the ores. These are broken from 
veins of quartz traversing slate and sandstone beds. When 
sent to the mill the quartz is accompanied b}^ a comparatively 
small admixture of country rock. The quartz is white, often 
honeycombed, and sometimes sugary. The gold which it 
carries is coarse, of very high caratage, often visible to the 
naked eye, and arranged for the most part along the faces of 
small fractures and seams traversing the quartz. A blow 
tends to readily detach the gold from the quartz. Occasion- 
ally the quantity of " mullock " or waste rock increases con- 
siderably, and the gold is accompanied by pyrites, chiefly 
arsenical iron sulphides, or occurs in a matrix composed of 
quartz and slate intermixed. 

Such, briefly, is the nature of the ore. The accompanying 
record, covering 14 years, will give a good idea of the com- 
pleteness of the extraction and the proportion of the value 
saved by the several contrivances which together make up 
the treatment. It is taken from the millbook of the Port 
Phillip & Colonial Company. 

THE PERCENTAGE OP GOLD OBTAINED. 



1865 
1866 
1867 
1868 
1869 
1870 
1871 
1872 
1878 
1874 
1875 
1876 
1877 
1878 
1879 



Beds.* 


Boxes. 


Blankets. 


Mills. 


68.60 


22.09 


10.55 


8.76 


65.60 


22.68 


8.78 


4.04 


66.44 


22.50 


8.48 


8.58 


68.22 


24.08 


8.11 


4.64 


61.88 


24.77 


8.02 


5.88 


60.15 


26.69 


2.74 


10.42 


62.59 


25.39 




12.02 


64.48 


21.60 


1.06 


12.86 


59.20 


20.67 


10.85 


9.78 


56.14 


22.54 


18.24 


8.48 


54.81 


25.14 


11.22 


8.88 


58.17 


21.24 


11.12 


9.47 


52.84 


21.56 


15.40 


10.20 


52.84 


17.12 


17.11 


12.98 


57.99 


10.50 


12.84 


18.67 



Blankets 
and mills. 



14.81 
12.77 
12.06 
12.75 
18.90 
18.16 
12.02 
18.92 
20.18 
21.82 
20.06 
20.69 
25.60 
80.04 
81.51 



Yield per ton. 




It will be noted that in 1870 the use of blankets was dis- 
continued and during 1873 it was again resumed. During 
the interval, it will be remarked, the yield from the Chilian 
mills, which treated the concentrated pyrites, increased con- 



• The terms " beds," " boxes," " mills/' correspond to " mortar boxes," " miUs," 
" Chilian mills," respectively. 



EARLY AUSTRALIAN METHODS. 115 

siderably. Looking down the first two columns it is seen that 
while the proportion of the total yield coming from the mor- 
tar boxes (or "beds") and wells (or "boxes") amounted to 
87.03 per cent from 1865 to 1868, it declined to an average of 
73.08 per cent, foi* the last four years. In 1879 it was only 
68.49 per cent. In the meantime the yield from the blanket- 
ings and concentrates (as shown by the product of the Chilian 
mills) increased correspondingly. The first two columns rep- 
resent that part of the gold contents which is essentially 
"free," while the third and fourth columns represent the 
remaining portion which is rebellious or "refractory." 

The explanation of the figures above referred to lies in the 
fact that the mine workings having become deeper, the ore, 
by the steady increase of the percentage of pyrites contained 
in it, had become less " free milling." On referring to the 
returns obtained from the pyrites this reasoning is confirmed, 
for while in 1866 the yield of concentrates amounted to 268 
tons, averaging 2 ozs. 19 dwts. 4 grs. from the crushing of 
59,578 tons, in 1879 the pyrites amounted to 421 tons, averag- 
ing 4 ozs. 15 dwts. 20 grs. resulting from the treatment of 
56,766 tons. 

To consider the methods in use : It will be allowed that in 
milling the use of costly chemicals is to be carefully avoided. 
Mercury is the one chemical most generally used. Since a 
large (55 per cent, to 65 per cent.) percentage of the gold in 
the ores treated at the Clunes mills can be arrested in the 
mortar boxes by the action of gravity alone, the practice of 
the district is altogether correct in so far as concerns the non- 
employment of mercury in the battery. When under the 
stamps mercury is always subject to "flouring" — that is, 
breaking of it into minute globules, which, collecting im- 
purities, become covered with a film, causing them to refuse 
to coalesce and so to be carried away with the tailings. 
Together with the direct loss of mercury there must always 
be also an indirect loss of gold, particles of which have 
entered into amalgamation with the escaping globules of 
mercury. 

The absence of copper or other amalgamating plates is 
remarkable. This also, keeping in view the character of the 
ore, is correct. Wells are excellent gold-saving appliances 



116 STAMP MILLIKG OF GOLD ORES. 

for ore of this type, in which the precious metal is both free 
and coarse. They require less attention, their first cost is 
less than that of amalgamating plates, and they are less 
affected by the occasional presence in the ore of minerals 
which are injurious to amalgamation. 

Blankets, when they are intelligently used, are also among 
the best of the simple contrivances known to the millman. 
Instead of having the bad arrangement, seen in so many 
mills, of giving them a width much less than that of the 
amalgamating tables or mortar boxes, which precede them, 
the blanket tables here have the full, uninterrupted width of 
two batteries. At the South Olunes United there is a clear 
blanket space of 10^ feet. 

Ordinarily the slope of the blanket strakes or tables would 
be from H to 1^ inches per foot, but at Clunes, owing to the 
employment of a larger quantity of water,* one is able to 
work them with an inclination of only three- fourths inch per 
foot. This is in itself an important factor, though apt to be 
overlooked. 

The after-treatment in the barrels may appear crude, but 
practice has shown that it is very effective. The bad custom, 
observable in some California mills, of putting pieces of iron 
into the barrel (with the idea of mixing up the pulp and 
grinding it) does not prevail at Clunes. It is a device which 
serves mainly to cause excessive loss of mercury which, quite 
as much as the pulp, becomes ground and so " floured." 

The double discharge (front and back) presents no striking 
features. It is successful in slightly increasing the crushing 
capacity of the mill, though it will be noted that it also re- 
quires the use of a much increased supply of water. 

The depth of discharge is a factor in milling the importance 
of which is almost invariably overlooked. The mills of this 
district are not guilty of the vicious practice of allowing a 
wide difference between the minimum and maximum depth 
of discharge, caused by the wearing down of the dies. An 
endeavor is made to keep the depth of discharge fairly con- 
stant, first by regulating the packing under the dies, and 
then by the placing below them (as they wear down) of a 
false bottom. 

* Due, as pointed out, to the use of the double discharge. 



BARLY AUSTRALIAN METHODS. 117 

Though the self-feeders used are not automatically perfect 
they do their work well, and, it is needlesss to add, are a 
great improvement upon the bad, irregular hand-feeding 
which prevails in the majority of colonial mills. 

The concentrating machinery may with reason be considered 
somewhat out of date, but the modified Cornish buddies in 
use are doing most excellent work, and it is doubtful whether 
they would be replaced to advantage by the more costly Frue 
vanner. 

Speaking generally, the treatment which the ore undergoes 
is remarkable, most of all on account of its simplicity, but so 
is the ore ; and in this way the practice of the district carries 
out the first postulate of intelligent milling, viz, that the 
treatment should be varied according to the character of the 
ore to be treated. 

After a careful examination of the ore mined at Olunes, 
and of the milling to which it is subject, it is not possible but 
to speak in words of commendation. To a millman Clunes is 
almost solitary among the gold-mining districts of the 
Colonies in being a quartz-milling center which does not leave 
a feeling of dissatisfaction and an impression of disappoint- 
ment. The old Port Phillip is still working, but as a great 
mining and milling establishment it is a thing of the past. 
That past has, however, been a glorious one, not so much by 
reason of the dividends which it has paid, but because of the 
successful experimental work which it carried on for so many 
years at a time when such work was especially needed. The 
immense good it has done as an educational center and a 
training ground for millmen is not known save to those 
acquainted with the work which was done at Clunes from 
1857 to 1880. You may visit mills in the most distant parts of 
Australia and almost without exception, wherever you find 
good, intelligent milling (and that does not happen too often 
to be monotonous) you will also learn that that knowledge 
and experience were obtained at the small Victorian town of 
whose record we are speaking. 

The Port Phillip was the first to introduce the system of 
taking daily assays as a check upon the work done in the mill. 
In this respect Clunes is still, unfortunately, a striking 
exception. 



118 STAMP MILLING OF GOLD OBSS. 

In another department this mill was almost a solitary 
pioneer. The rock-breaker was introduced by the Port Phillip 
in 1865. Can it be believed that in these days of improved 
milling machinery, when the rock-breaker is accepted as an 
absolutely necessary portion of a complete mill equipment, 
that in the great gold mining colony of Victoria there are 
only 12 rock-breakers ? Of this number three are accounted 
for by Olunes, two belonging to the Port Phillip, and one to 
the Dixon's North Clunes. 

In closing this short account of the milling practice of a dis- 
trict but little known beyond the colonies, it will be pardoned 
if I express the opinion that the work done at the Port 
Phillip & Colonial Company's mill has been of more wide- 
reaching usefulness and more permanent benefit to the mining 
industry of Australia and New Zealand than that of any other 
company which has gone into operation since the days of the 
discovery of gold. I wish to record my conviction of the 
debt which quartz milling in the Colonies owes to the 
manager, Mr. K. H. Bland, of the Port Phillip, who started 
the operations in 1866, conducted the numerous and valuable 
experiments which did so much to establish the correct basis 
of milling practice, and to-day still assists the industry by 
his sterling good sense. 



CHAPTER VIII. 

More Modern Australian Methods. 

When gold was first found at Golden Point in August, 1851, 
the history of Ballarat began. It marked the commencement 
of a record more brilliant than that of any other of the great 
discoveries of that golden year. To-day Ballarat is the 
largest gold-producing mining district in Victoria, for, unlike 
many of the rich diggings of the early days, it has not yet 
outlived its first reputation. The working of the rich alluvium 
commenced the development of the mineral resources of 
Victoria, and laid the basis of the commercial wealth of Mel- 
bourne. That alluvium, however, has been largely exhausted 
and at the present time more than half the gold product of 
Ballarat is derived from the quartz lodes. It is a remarkable 
fact in the history of mining that mines which had produced 
largely from the alluvium became no less profitable as quartz 
mines, for in the bedrock of the deep leads were found the 
croppings of the quartz veins whose erosion in a bygone 
period had enriched those deep leads.* 

For the year 1891f the output of the Ballarat district 
amounted to 202,740 ozs. 1 dwt. 12 grs., of which 74,768 ozs. 13 
dwts. 3 grs. were alluvial and 127,971 ozs. 8 dwts. 9 grs. came 
from the quartz. Dividends amounting to £222,839 15s. Od. 
were paid during the twelve months. The average yield of 
the quartz was at the rate of 8 dwts. 1 gr. per ton, while the 
pyrites (concentrates) contained 2 ozs. 3 dwts. 2 grs. per ton. 
The price of the gold varied from £3 17s. 6d. to £4: 3s. Od. per 
ounce. I At the present time there are at work in the district 

* At the Band and Albion 519^1 ounces, worth £2,078,285, were obtained from the 
alluvium up to 1879, when the value of the quartz reef in the bedrock was recog- 
nized. Quartz succeeded alluvial mining, and gold of the value of £607,880 was 
obtained up to June, 1888. Total gold obtained, £2,685,615; dividends, £1,415,588. 

f For 1894 the total output from Ballarat was 194,820 ozs. 6 dwts. 12 grs., of which 
76,269 ozs. 8 dwts. 8 grs. were alluvial, and 118,551 ozs. 2 dwts. 9 grs. came from the 
quartz. The dividends paid amounted to £151,269 Os. 5d. 

I For these figures the writer is indebted to the Annual Report of the Secretary 

for Mines. 

(119) 



MORE MODERN AtlSTRALIAX METHODS. 121 

1,246 etampheads, 21 buddies, 4 stone-breakers, 28 concentrat- 
ing tables, 2 Wheeler pans, 11 Berdans, 7 Chilian mills, and 
6 Arastras. The average price paid for crushing quartz is & 
shillings per ton. A 5-head battery can be hired at the rate 
of 2i shillings per hour. 

The comparative table which follows will give figures illus- 
trating the chief features of the methods of work employed 
at four of the principal mills : 

COMPAEATIVE TABLE. 



The ytar of the East at Sebastopolt was for many years the 
most productive gold-quarta mine in Victoria.§ The ore is 
treated at the two mills, one of which contains 60 stamps and 
the other 20. The larger plant is only two years old, while 
the smaller dates seven years back. 

Going to the new mill first it is found that it consists of 60 
stamps in two 30-head sections. Each stamp weighs slightly 
over 1000 pounds and drops 73 times per minute. The height 
of the drop varies from 8 to 8i inches. The center stamp of 
a battery of 5 heads drops one-half inch lefs than the other 
four. This is said to produce a better splash of the pulp 
against the grating or screen. The order of the drop is 5, 3, 
4, 2, 1. The discs or tappets are of screw pattern. The depth 

*Longtoas, 2,Ml pounds. 

+ A flask of mercury in tbe ooloDlea oontalna 75 pounds avoirdupois, 
t Ballsrat was born during tbe time of the Crimean War, and the names of 
former battleflelda adorn e^ery street corner, 
f During leei this mine produced 34.003 ounces gold and paid £79,200 in dividends. 



122 STAMP MILLING OF GOLD ORES. 

of discharge or issue — the distance from the bottom of the 
grating to the top of the false bottom or die — is 2 inches 
when starting with new dies. Sand is packed tightly under- 
neath. As the dies wear down a maximum depth of dis- 
charge of 4 inches is attained. When the gold in the ore is 
found to be unusually fine the dies are allowed to wear down 
still further so as to obtain as deep an issue as possible. The 
capacity of the mill is at the rate of 1,450 long tons per fort- 
night of 12 working days. The grating or screen is of round- 
punched Eussia iron having 200 holes per square inch. Such 
gratings last for 12 days. On rare occasions, when ore con- 
taining gold in an unusually finely divided condition is en- 
countered in the mine, gratings having 270 holes per square 
inch are substituted and their time of wear is only 3 to 5 
days. As a rule the gold is fairly coarse, though there is a 
marked difference in this respect between the product of the 
two lodes being worked in the mine. The percentage of con- 
centrates obtained is 3^. They contain from 3 ozs. 5 dwts. to 
3 ozs. 13 dwts. per ton, and are sent for treatment to the 
chlorination works at Bendigo.* 

The bullion is very high grade, averaging 970 fine. The 
percentage of bullion obtained in retorting is found to vary 
with the ore broken from the two lodes. The amalgam from 
the No. 2 shaft retorts 45 per cent. ; from the No. 1, 48 per 
cent. The grade of the ore is in accordance with the retort 
yield, viz, 13 and 17 dwts. per ton, respectively. This is in 
harmony with the usual experience that rich ore, because the 
gold is also usually more coarse, yields higher grade amalgam. 

The amount of water used is at the rate of 7^ gallons per 
stamp per minute, not including the boiler supply. In one 
month 16,588,800 gallons are consumed. The loss of mercury 
amounts to one bottle, or 75 pounds avoirdupois, per month. 
This includes that consumed at the old mill and is at the rate 
of 6.7 dwts. troy per ton of ore crushed. 

In this mill the gold saving is done in the mortar box, 
by copper plates outside, by wells or mercury traps, and 
indirectly by the blanket strakes and shaking tables. The 
mortar box is made an amalgamator by the introduction 

* since the time when the writer was at Ballarat a new chlorination plant has 
been erected in the town itself. 



MORE MODERN AUSTRALIAN METHODS. 123 

every two hours of a teaspoonful (4 ounces) of mercury. 
There are no amalgamating plates inside. The amalgamating 
tables outside are covered with sheets of plain copper. They 
have a grade of seven-eighths inch per foot. At their lower 
end there are two drop-wells and one shallow well. These 
catch but very little gold; indeed, when the copper plates 
above them are in good order they simply serve to arrest 
escaping mercury. Next come the blankets. The blanket- 
ings — the residues from the regular washing of the blankets — 
were formerly treated in an amalgamating barrel, but now 
they are stacked, lime is added and they are allowed to stand 
for two days, after which they are fed into the battery in 
company with the usual ore supply. Below the blanket 
strakes are 8 ordinary shaking tables of inferior pattern. 
They do not discharge the concentrates automatically. The 
pulp which escapes from these passes over "tyes" or straight 
sluices. Three Berdans are used for grinding the skimmings 
from the wells. 

The old mill contains 20 heads, weighing 785 pounds each. 
It has a crushing capacity of 350 long tons per fortnight of 
12 working days. The arrangement of the different parts is 
very similar to that of the new mill. The copper amalga- 
mating tables incline at three-fourths inch per foot. They^ are 
preceded by two, and followed by three, wells. Then come 
two strips of blanket having a total length of 8 feet and a 
gradient of H inches per foot. These are succeeded by four 
shaking tables, one to each battery. Two Berdans are used 
in the treatment of skimmings. A ball is first used and then 
a stationary drag, the amalgamation being carried on during 
both stages. It would be better to do the grinding of the 
material first with the ball and then add the mercury, and do 
the amalgamation during the supplementary grinding done 
by the drag. The drag being stationary would not work 
upon the quicksilver ; the former is placed at the side of the 
pan and the latter settles at the lower end. Less mercury 
would then be lost by flouring. 

The two mills compared. — The working of an old plant side 
by side with a new one treating the same ore naturally in- 
vites a comparison, which in this instance is prejudicial to the 
newer, larger, and more costly mill. The millman acknowl- 



^ n - 



'V 



124 STAMP MILLING OF GOLD ORES. 

edges that the older mill is doing the better work. In this he 
is, I believe, correct. The main distinction between the two 
consists in the fact that in the 60-stamp mill the copper 
plates do that part of the gold saving which in the 20-stamp 
mill is accomplished by wells. In both cases the bulk of the 
gold does not go further than the mortar box itself, but is 
arrested by the action of gravity aided by the free mercury 
introduced by the feeder. Of thaft which escapes from the 
battery the plates get the larger portion in the new mill, but 
in the old one it is obtained by the two wells which precede 
the copper plates. Which is the better arrangement ? The 
plates have a greater first cost and they require far more 
attention than the wells. It is only in comparatively excep- 
tional cases, where the ore does not contain a large percentage 
of sulphurets and where the gold is comparatively "free," 
that wells can be satisfactorily used instead of plates, but 
where, as in this instance, they are found to accomplish the 
work of arresting the gold they are for the above-stated 
reasons much to be preferred. 

In both mills the feeding is done, and done badly, by hand. 
Four laborers (three young fellows and one man) are em- 
ployed per 8-hour shift in keeping the 60 heads supplied. 

It is the custom to add a bucketful of lime to each 10 heads 
every two hours, since this is found to prevent the formation 
upon the amalgamating tables of a black scum due to the 
presence in the ore of sulphated pyrite. 

By way of further criticism it is not too much to say that 
it is but a sorry result that with such a large and productive 
mine, situated in a very enterprising mining district, the new 
mill should be so far behind the ideal. There is absolutely 
no excuse for the want of a rock-breaker and the absence of 
proper feeding machines. The cost of such an installation 
cannot be pleaded by one of the wealthiest mining companies 
of Australia. Fault must also be found with the concentrat- 
ing machinery, which is altogether inadequate to the require- 
ments of the mill. An increase is wanted in the number of 
shaking tables, but in making any addition it would be well 
to secure percussion tables of proper design, discharging the 
concentrates automatical! v. 



MORE MODERN AUSTRALIAN METHODS. 125 

At the Britannia United on Bakery Hill* there are 60 
stamps. The most important figures indicative of the method 
of milling are given in the comparative table. The ore is 
somewhat more free milling than that of the Star of the East, 
as is shown by the lower (1 per cent.) percentage of concen- 
trates, the greater fineness of the gold, and the increased 
retort yield.f The bullion is of an unusually high caratage, 
being worth £4 3s. Od. per ounce J or 978 fine. The coarse- 
ness of the gold in the ore is shown by the retort percentage, 
which, while it sometimes reaches 66, averages 50. The gold 
saving is effected by methods similar to those described at the 
60-stamp mill of the Star of the East company. One ounce 
of mercury is added to the mortar box per ounce of gold in 
the ore. Immediately outside the battery there is a well 1^ 
inches deep and 3 inches wide, containing 10 pounds of mer- 
cury. The copper plates have a grade of 1 inch per foot. 
The blanket strakes are 16 feet long, in three longitudinal 
divisions each 17 inches wide. They have a slope of 1^ inches 
per foot. Quicklime is added to the battery at the rate of 5 ^^(^tt. 
pounds per each 5 heads per 24 hours. The water used in the -^v^^^j. _ 
batteries is warm and is made so by conducting the condenser ^*^ 

water of the engine into the tank which supplies the mill. 

Two points open to discussion are here suggested, viz, the 
use of warm water and the addition of lime. The object of 
heating the battery water in such a warm climate as that of 
Ballarat does not appear very evident. The use of condenser 
water in any mill is decidedly objectionable. To consider 
these two propositions, let us take first the effects of warm 
water upon amalgamation. At the alluvial mines of the 
mountains of the interior of Otago, New Zealand, the use of 
mercury,§ the good friend of the miner all the world over, is 
hardly known, and the explanation given is that mercury will 
not act in the cold climate of that region. This is due to the 
use of hot water in cleaning up at both mines and mills. 
The idea is, of course, quite an erroneous one, though there is 

* Near the spot where was found, on June 15, 1858, the Welcome nugget, which 
was sold by the lucky discoverers for £10,500. It weighed 2,159 ounces. 

+ Also the coarser crushing required to liberate the gold. 

X Pure gold is worth £4 4s. 11 ^d per ounce at the London mint. 

^Discussed by the writer in "Alluvial Mining in Otago." Trans. American 
Institute Mining Engineers, Vol. XXI. 



126 STAMP MILLING OF GOLD 0RB8. 

a substratum of truth in it from the fact that amalgamation 
is usually assisted by heat and retarded by cold, but within 
narrow limits only.* On the other hand, at Black Hawk, at 
over 8,000 feet above the sea level, in the bitter cold of the 
Colorado winters, the millmen will tell you that cold weather 
is better for amalgamation upon the plates than summer heat. 
Why? Because heat thins the amalgam, and the vibration 
of the mill due to the falling stamps causes the globules of 
mercury to run off and down the surface of the amalgamating 
tables, while cold (which thickens the amalgam) tends to 
keep it in position. From one point of view hot water is to 
be recommended. Slimes which will float on cold water will 
sink in warm water, owing to the expansion of the air bubbles, 
which float the fine dust and are the raison cTetre of the 
slimes. On the whole, however, while amalgamation (and 
here the amalgamation of gold is the only question discussed) 
is assisted by heat, yet below the temperature of boiling water 
the effects of a small rise are so slight that it is doubtful if 
the use of warm water is to be advised in ordinary gold stamp 
milling. It is certainly not to be recommended in summer at 
a locality having the climate of Ballarat, and therefore its 
use at the Britannia United is to be objected to.f 

It is safe to say that the employment of condenser water is 
altogether objectionable. On suggesting that such water 
would be sure to carry grease with it the millman dis- 
agreed. At the New Ohum Consolidated at Bendigo, where 
the condenser water was not used in the mill, I examined the 
launder which carried it outside the building and found that 
the bottom and sides were coated with a slimy ooze which 
could not but be prejudicial to amalgamation. Grease of any 
kind is the millman's worst enemy, for it coats the globules 
of mercury and prevents their coalescing ; it makes " flouring,'^ 
which is the subdivisioti of mercury into small particles, 
permanent. Yet at this (Britannia United) mill, where con- 
denser water is used, the loss of mercury per ton of ore crushed 
is only 2.7 dwts., unusually small, and less than at the 

*The amalgamation of gold, not silver, in ordinary stamp milling, not pans, is 
here discussed. 

t At the time referred to it was summer and the temperature outside the mill 
82© P. 







MORE MODERN AUSTRALIAN METHODS. 127 

neighboring Star of the East mill, where condenser water is 
not used. The explanation of these apparently contradictory- 
facts is to be found in the addition of quicklime. 

Five pounds of quicklime are added every 24 hours to each ^^ 
battery of five heads. As an alkali it is a solvent for grease, o. 
and though not intended as an antidote for the greasy matter ^-j 
contained in the condenser water, there is no doubt that it ^ < 

acts as such. 

At the Star of the East it is added to the blanketings pre- 
vious to their reintroduction into the battery ; it is also, as at 
the Britannia United, added directly with the ore fed into the 
battery. It is, indeed, in general use at the Ballarat mills 
with the purpose of keeping the amalgamating tables in good 
order. Lime as an alkali serves to neutralize the acidity of 
the battery water (produced by the reactions upon the par- 
tially oxidized sulphides when under the stamps), and in 
this way prevents the corrosive action of such water upon 
the iron of the gratings and the copper of the tables. 

At the New Normanby mill in Ballarat East* the ore 
crushed is of a very free-milling character ; it occurs coarse 
and in quartz almost free from pyrites. There is no concen- 
trating machinery. The character of the gold is proved by 
the percentage obtained in retorting the amalgam, the yield 
of bullion being rarely under 55 per cent, and averaging very 
nearly 70 per cent. Four-fifths of the product of the mill 
comes from the mortar box, and of the remainder nearly all 
is caught by the first narrow strip of copper plate and the 
well immediately below it. The blanket washings are very 
poor. 

The North Cornish mill is not in the town of Ballarat, 
though in the mining district of that name, but at Dayles- 
ford. The plant consists of 50 stamps, and the rate of drop 
varies from 70 to 75 per minute, the height of the drop being 
8 inches. The depth of discharge or issue is extremely irreg- 
ular. When starting with new dies or false bottoms the top 
of the die is approximately level with the bottom of the 
grating and the depth of discharge is practically nil, but as 
the dies wear down (they are 4 inches deep) the issue in- 

* Near the famous Canadian lead, which yielded sq many of the large nuggets. 



128 STAMP MILLING OF GOLD ORES. 

creases to a maximum of a little over 3 inches. The mill 
crushes 90 long tons per day, or 2,450 tons per month.* 

The amalgamation is effected by methods similar to those 
described as in use at the Star of the East. Scarcely 11 per 
cent, of the amalgam obtained comes from the mortar boxes. 
Much the largest proportion is derived from the skimmings of 
the wells and in the blanket sands. The latter are treated 
by Berdan pans, of which there are six. The tailings from 
the Berdans go to a Frue vanner for after-treatment. 

This is a comparatively new mill, the oldest portion being 
five and the newest two years old. The quantity of ore in 
sight in the mine is such as to have led to the consideration 
of a further addition to the number of stamps. The mine 
has paid large dividends for alongperiod.f Notwithstanding 
these facts the mill is miserably incomplete. Although very 
favorably situated and having all the fall required for an 
automatic arrangement of its parts, it is unprovided with rock- 
breakers or self-feeders. In the feeding of the 50 stamps 
there are employed upon each shift 6 men at 80 shillings per 
week, making a total cost of JC1,125 per year. The other end 
of the treatment shows equally grave defects. In spite of 
the fact that the concentrates are of unusual richness and 
that to them the shareholders practically owe their dividends, 
there are only 14 Frue vanners to treat the pulp coming from 
50 stamps. The ore is one requiring very careful concentra- 
tion, since after having passed over the amalgamating tables 
it still carries black slimes known to be very rich. For the 
saving of such material the Frue vanner is excellently 
adapted, but it is a machine which must not be crowded, and 
for the work to be done at the North Cornish mill 22 of these 
concentrators is the least number that can be employed con- 
sistently with good work. 

On turning back to the comparative table it will be noted 
that the five mills whose figures are given show a general 
resemblance in their methods. They fairly represent existing 
colonial practice. It will be seen that the old Star of the 

* For the half year ending December 81, 1890, there were 14,664 tons crashed, 
yielding 4,886 oz. 18 dwts. in the mill, and 1,486 oz. 5 dwts. from the 269K tons of 
pyrites (concentrates) treated at the chlorination works. 

+ Up to the end of 1890, on a paid-up capital of only £4,000, this property has paid 
dividends amounting to no less than £85,500. 



MORS MODERN AUSTRALIAN METHODS. 129 

East mill, the New Normanby, and the North Cornish all have 
the same weight of stamp. The other two have unusually 
heavy stamps, and in this respect they are following the 
tendency which was to have been noted in California a few 
years ago, and like the California mills they, too, will probably 
ere long find it expedient to revert to a lighter pattern. This 
is an instance of the same ground being gone over twice and 
of the needless expenditure of time and money in trying 
experiments which some other district has already carried out. 
There is no worse waste than the waste of experience. 

The Britannia United and the New Normanby make 12 
drops per minute less than the other three, owing to the fact 
that the gold being coarser and more free the ore does not 
necessitate fine crushing. The fineness of the gratings indi- 
cates this. The height of the drop is practically uniform save 
in the case of the New Normanby, where, owing to the 
unusual coarseness of the gold and the absence of pyrites 
worthy of concentration, rougher stamping is permissible. 

In the matter of the issue or depth of discharge there is 
that wide variation usually to be remarked in most of the 
colonial mills. It is a feature of milling the importance of 
which is too little appreciated all the world over. At the 
Britannia United and New Normanby mills there is an effort 
made to prevent too great a variation in the issue as the dies 
wear down, but at the North Cornish the difference is between 
zero and 3 inches. The effect of a shallow discharge is seen 
at the Britannia United, which, notwithstanding a much 
slower speed, has a relatively higher crushing capacity than 
the Star of the East. 

The gratings or screens are all made of the same material, 
round-punched Russia iron. The coarsest sizes are naturally 
in use at the New Normanby and the Britannia United, the 
two mills whose ore carries the coarsest gold and in the most 
" free " condition. The North Cornish uses the finest grating, 
since the gold of the ore which it treats is the most intimately 
associated with the pyrites. 

The simple character of the gold quartz of this district is 
very well evidenced by the coarseness of the crushing which 
it permits of. 

In the matter of screens the Colonial mills have been keep- 



130 STAMP MILLING OF GOLD ORES. 

ing to that follow-my-leader policy which is the keynote to 
all that is deficient in their modes of milling. A little con- 
sideration will lead one to the conclusion that wire cloth has 
a much larger area of discharge than punched iron, having 
openings of equivalent size. Actual practical tests have con- 
firmed this deduction. The crushing capacity of many of the 
Ballarat mills would be increased 10 to 20 per cent, by the 
use of wire-cloth gratings, and the amalgamation would be in 
no way altered or increased. The little extra cost is much 
more than compensated for by the larger amount of ore 
which can be treated. The full benefit of the change can be 
best obtained by having a double set of gratings, so that 
while one set is in place in the battery the other can be dried 
and cleaned with wire brushes. 

In the matter of concentrates it will have been remarked 
that the New Normanby ore carries no pyrites worthy of con- 
centration, that the Britannia United mill obtains only 1 per 
cent., and even that is of comparatively low grade, while the 
North Cornish, which has much the most refractory ore, has 
also the richest concentrates. It should be added, however, 
that the closer work done by the Frue vanners as compared 
to that of the ordinary percussion tables at the Britannia 
United, assists in keeping up the grade of the concentrates 
obtained at the former mill. The fineness of the bullion tells 
the same story as the concentrates, that of the Ballarat mills 
being of unusual purity and better than that of the Dayles- 
ford mill. The retort percentage indicates more accurately 
than any of the other figures the character of the ore as 
regards the coarseness of the gold which it carries. Thus the 
New Normanby amalgam yields 70 per cent, of bullion, while 
the North Cornish produces the more ordinary proportion of 
33 per cent. 

The time of wear of the screens or gratings is very similar 
in the first four mills, but at the North Cornish it is decreased 
to nearly one-half. The explanation is not far to seek. It is 
to be found in a very shallow, but variable, depth of discharge. 
When new dies have just been placed in position the top of 
them is level with the bottom of the screen and the direct 
splash of the pulp is, of course, violent. The discharge is then 
allowed to increase to several inches. It would certainly be 



MORE MODERN AUSTRALIAN METHODS. 131 

well to make an effort to regulate the issue and to keep it 
more uniform. At present it varies from zero to 3 inches. It 
is probable that the extraction of the gold would be benefited 
by leaning rather toward the greater than the lesser depth of 
discharge. At the Britannia United the average depth of the 
issue is no greater, but the variation is between 1 inch and 2 
inches. This explains why the gratings, assisted by the fact 
that they are also less fine, have a time of wear double that 
at the North Cornish. 

The consumption of mercury is much less at the New Nor- 
manby and Britannia United mills than at the other three, 
because these two treat ore containing a minimum percentage 
of pyrites, and because, therefore, the crushing is less fine. 
The amount of water used depends upon the grade of the 
amalgamating tables, itself proportioned to the heaviness of 
the pulp. It is also largely dependent upon the extent of 
blanket surface. The North Cornish uses much less water 
than the others, because the blankets, being followed by Frue 
vanners, are shorter than at the other mills where simple per- 
cussion tables are used. 

The consideration of the methods of this district lead one 
to the conclusion that the mills are not availing themselves 
as they ought to, and as was to have been expected from so 
energetic and so productive a mining center, of the improve- 
ments brought into successful practice during the last decade. 
At Ballarat there is neither want of capital nor an inadequate 
ore supply to excuse the miserable incompleteness of the 
mills, in so far as concerns appliances and arrangements having 
in view the automatic handling of the ore. For mining com- 
panies like the Star of the East and the North Cornish, both 
owning magnificent mines paying large and regular dividends, 
and possessing very considerable ore reserves, there can be 
no excuse for the non-employment of rock-breakers, ore-feed- 
ing machines, and a proper and adequate concentrating plant. 
They stand as monuments of what should be more truly called 
obstinate ignorance and perverse disregard of modern experi- 
ence than dignified by such a word as " conservatism." It is 
very regrettable that for reasons, all of them illogical and 
untenable, the mills of such an important mining district 
should be so out of date and so incomplete. 



132 STAMP MILLING OF GOLD ORES. 

In conclusion, therefore, it must be said that while the 
actual work is excellently well carried out, the mills of Bal- 
larat are woefully behind the ideal both in that handling of 
the 'ore which immediately precedes stamping and in that 
after-treatment which succeeds amalgamation. 



CHAPTER IX. 
Gold Milling at Bendigo, Victoria. 

This district has had the misfortune of undergoing more 
than one christening, which explains the fact that it is not so 
well known in the northern hemisphere as its neighbor 
Ballarat. It was called Bendigo when in the autumn of 1851 
it was changed from a sleepy sheep run to an excited mining 
camp, but the first name subsequently gave way to the more 
English Sandhurst, by which it was known for several decades, 
until in 1891, by a general consensus of opinion, the original 
name was resumed. 

Though the riches of the first-found alluvium were exhausted 
at a comparatively early period, the later discovery of gold- 
bearing quartz lodes of great value, of unusual permanency 
and of very peculiar structure, has made the Bendigo district 
the greatest *' quartz-reefing" center of Australia. This gold 
field now contains 26 mines having shafts exceeding 2000 feet 
in vertical depth. Six shafts are more than 2500 feet, and one 
(Lansell's 180 mine) has recently passed the 3000-foot mark.* 

Since the date of its discovery in November, 1851, this 
district has yielded 12,000,000 ounces of the precious metal, 
valued at ie48,000,000 sterling, or $240,000,000. 

In 1892 the Bendigo district produced 198,009 ozs. 4 dwts. 
2 grs., of which amount 5,750 oz. 2 dwts. grs were of alluvial 
origin, and 192,259 ozs. 2 dwts. 2 grs. came from the quartz 
reefs. During that year the dividends paid amounted to 
iC268,2()3, being at the rate of $6.50 per ounce of gold produced, 
or over 30 per cent. 

The average yield of gold per ton of quartz ore was 9 dwts. 

* Since this was written these shafts have been deepened and there are now 

several beyond the 8000-foot mark, the 180 mine having reached a depth of 8850 

feet. 

(188) 



134 STAMP MILLING OF GOLD ORES. 

23 grs., and the average contents of the pyrites or concentrates, 
2 ozs. 5 dwts. 8 grs. The price of the gold bullion varied from 
£3 17s. Od. to £3 19s. Od. per ounce. There were at work in 
1892, according to the report of the Secretary of Mines for 
Victoria, in this district, 1279 stamps, 105 concentrating 
tables, 8 Chilian mills, and 32 arastras. 

The comparative table which follows will give the chief 
figures indicative of the mode of treatment at a number of 
the most representative mills in the district. 

The Fortuna " crushing works " are the property of Mr. 
George Lansell, the leading mine-owner of the district. This 
mill is engaged in treating the ore coming from properties 
belonging to the proprietor, such as the "180," "222," and 
Comet mines, of others in which he holds a large interest, such 
as the Lazarus, and in doing a general custom business. 

The plant consists of two portions, containing 48 stamp 
heads in all. Six mortars, of five heads, each are compara- 
tively new, dating to 1888, while the remaining 18, in three 
mortars of six heads each, are old. The stamps weigh 900 
pounds each, this total weight being distributed as follows : 
Disk or tappet, 66 pounds ; shank or stem, 325 ; tophead or 
boss, 159 ; head or shoe, 198 ; false bottom or die, 152 pounds. 
The stem is 12^ feet long ; 8 feet of 3i inches, and 4^ feet of 
2| inches iron. 

The speed averages 70 drops per minute, and varies from 
68 to 75. The height of the drop varies from 8 to 9 inches. 
At the time of the introduction of new dies the issue or depth 
of discharge is 2 inches, but this increases as the dies wear 
away to a maximum of 6 inches. 

The shoes are 10 inches high by 9| inches in diameter. The 
dies are 5 inches high. Steel shoes and dies were tried and 
were found to wear well, but being imported from England 
at a cost of £30 per ton, it was found more economical to use 
locally made cast-iron shoes and wrought-iron dies at a cost 
of £12 per ton for either. The wear per ton of ore is 16.4 
ounces of the shoe and 6.2 ounces of the die. The tappets are 
screwed on ; no keys are used. The order of the drop is not 
well adjusted, and approximates 5 and 1, 2 and 4, 3. 

The crushing capacity of the mill is at the rate of 360 loads 
per week for the 30 heads, and 240 loads for the 18 heads. 



GOLD MILLINa AT BENDIGO, VICTORIA. 



185 



The week has only six working days. A load of quartz ore 
is about equal to a ton of 2240 pounds. The rate per stamp 
is therefore two tons per day in the new section and slightly 
over that figure in the old part of the mill. 



COMPARATIVE TABLE. 

















S 




•8 








p< 




• 




d 


■»a 


d 


Name of mill. 


OQ 
P. 

a 

00 




• 

s 

o 


r of drops 
;e. 


• 

P. 
o 


oS 

1 


y of stamp. 


o 


Vi 
O 

c 
o 


er square 1 
ting. 






■a 


umbei 
minut 


■a 





4^ 


apacit 
hours. 


escrip 
ing. 


:oles p 
in gra 




» 


» 


w 


p 


o 


O 


P 


pq 






Lbs. 




In. 


In. 


Tons. 


Tons.+ 






New Chum Consolidated 


80 


805 


72 





8^ 


2.0 


60 


•o 


170 


LadyBarkly* 


40 


966 


68 





2.1 


84 




115 


Pearl 


ao 


840 


74 


8^ 


2.2 


66 


flcn . 


168 


Fortuna 


48 
64 


000 

U50 

jooo 


70 
60 


4 
8^ 


2.1 
2.1 


100 
184 


S d <"> A 
Op0 


143 


Catherine] 


143 

1 



Name of mill. 



New Chum Consolidated 

Lady Barkly* 

Pearl 

Fortuna 

Catherinell 



I 

d 

d 
o 



9 

bo 

^ • 
d « 

O 08 
»^ h 

©♦* 

(1h 



Per ct 



1J6 



H 



L< * 

^d 



0? 

d 

8 



9 

o 

d 
o 
o 

Sd 



^ 



Oz4 Dwts. 



2 11 

1 15 

2 8 
Various. 

3 4 









i!. 


• 






« 


d 
o 

^-« 



OQ 


d 

9 


• 

d 

i 


a 

o 

d 
o 

■Id 


OQ 


P* 
^ 





a . 


d 


U 


u 


sb 


d 


2 


OS 


OQ h 

CIS 


w 
fo 


& 


^ 




Per 1000. 


Per ct. 


Days.$ 


Dwts. 


056 


46 


17 


7.76 


041 


54 


12 


0.5 


054 


66 


10 


6.7 


068 


56 


7H 


8.8 


052 


66 





7.8 



a 

03 

OQ 
U 

DD d 

Sa 



Gals. 
** 

6! 



4v; 



The gratings or screens used are variously made of iron and 
steel plate punched with from 120 to 180 holes per square 
inch. A grating having 143 holes per square inch, and made 
by Goyere & Co. at the neighboring town of Epsom, is the 
one most used. The holes are 23-1000 inch in diameter. 
In the 30-head section the gratings have an average life of 

*Full name, ''South St. Mungo and LadyBarkly.'' 

f Long tons, 2,240 pounds each. 

X Sil\er contents negligible. 

II Full name, " Catherine Reef United. 

$ Working days of 24 hours each. 

** No figures obtainable. 



136 STAMP MILLING OF GOLD ORES. 

nine days, but in the old 18-head section this is diminished to 
six days. The stamps in the older portion of the mill are 
nearer the grating frame ; hence the discharge of the pulp is 
more rapid, giving increased crushing capacity, but a diminu- 
tion in the life of the gratings, which are subjected to a more 
violent abrasion by the pulp in the mortar box. 

The percentage and value of the concentrates are not given 
because the ore supply comes from a number of mines and 
has a very variable composition. In 1890, 313 loads of pyrites 
were obtained and 20,582 tons of quartz crushed. A load of 
concentrates weighs from 25 to 30 hundredweights, so that the 
above amount is equivalent to, say, 460 tons. This does not, 
however, in any way properly represent the amount of pyrites 
in the ore, since the concentrating apparatus was at that time 
very incomplete. The actual percentage of pyrites in the 
quartz varies from ^ to 3 per cent. 

The ore is fed into the battery by hand. Automatic feeding 
machines are not used in Bendigo, nor are rock-breakers. 

The gold saving is done by the mortar box, by amalgamat- 
ing tables, by wells, and indirectly by blankets. Mercury is 
added to the ore in the battery at the rate of 3 to 4 ounces 
per 5 heads per day when crushing 8 to 10 pennyweights ore. 
More is added when treating richer ore. 

The amalgamating tables are covered with sheets of plain, 
not silver-plated, copper. In the old section they slope at 1 5-16 
inches per foot, while in the new section the gradient is 1^ 
inches per foot. The tables are 5 feet wide and 13 feet long. 
There are five wells in all ; the lowermost three are shallow 
and do not contain mercury. The other two, at the head of 
the amalgamating tables, are 3 inches deep and 2^ inches wide. 
The blanket strakes are 5 feet in the clear, divided into 
three strips, and have a length of 15 feet. In the 30-head 
section they slope at H inches per foot, in the old section 
1 5-16 inches. The blankets are washed at an average interval 
of two hours. At the time of my last visit to the mill no 
further attempt at concentration was attempted, but since 
then the enterprising owner has introduced nine Gilpin 
County (Colorado) shaking tables, which are doing excellent 
work. The mill at night is lit up by electricity. 
The Catherine Reef United, at Eaglehawk, is the oldest 



GOLD MILLING AT BBNDIGO, VICTORIA. 137 

mill of the five whose figures are given in the comparative 
table. It contains 64 stamps. The weight varies. Thirty 
heads arranged in six batteries weigh 900 pounds, so also do 
24 heads in four batteries of six stamps each, while the 
remaining ten, divided into two batteries, weigh only 750 
pounds each. The heavier stamps crush 2^ tons each per 24 
hours, but the light section only 2 tons. 

The speed is regulated at from 68 to 70 drops per minute. 
The height of the drop is 9 inches, and is kept fairly constant. 
The depth of discharge or distance from the bottom of the 
grating to the top of the die is measured by 2 inches when new 
dies have been placed in position, and increases to a maximum 
of 5 inches as they wear down. The dies or false bottoms are 
4 inches deep and 10^ inches in diameter. 

The grating in ordinary use has 143 holes per square inch. 
The 180 and 200 sizes are occasionally used. Their average 
life is nine working days. 

The gold saving is done in the mortar boxes, to which 
mercury is added, by the wells and the copper plates, supple- 
mented by blanket strakes. There is no concentration proper 
beyond the saving of the heaviest pyrites by blankets. 

The tailings, owing to the lack of natural fall, are pumped 
into settling-pits. 

Eight bottles of mercury, each containing 75 pounds avoir- 
dupois, are consumed in a year, or at the rate of 7.3 penny- 
weights per ton of ore crushed. The mill only works during 
the daytime — 16 hours out of the 24. In six months 19,550 
tons were treated. 

The ore comes from a spur formation in the Catherine Reef 
United mine. The gold occurs in the quartz in a coarse and 
generally free condition. The average mill return is as fol- 
lows : In a fortnight of 12 working days 786 tons were crushed 
and yielded 146 ozs. 7 dwts. of bullion, together with 4 tons of 
blanketings carrying 12 ozs. 18 dwts. The gold has a carat- 
age of 22, 3, 2, equivalent to about .952 fine; 250 gallons of 
water are used per stamp per hour. 

The Lady Barkly mill is the joint property of the Lady 
Barkly and the South St. Mungo mining companies. Each 
company operates half the mill, which consists of two sec- 
tions of 20 stamps. Each stamp weighs 9 hundredweights. 



138 STAMP MILLING OF GOLD ORES. 

The stem weighs 336, the tappet 60, the boss 236, the shoe 196, 
and the die 109 pounds. The mill crushes at the rate of 84 
tons per day. Twenty heads in 12 days (a fortnight), work- 
ing 16 hours per day, put through 335 tons. 

The gold saving is done by the mortar box, by copper-plated 
amalgamating tables, by wells, and indirectly by concentrators. 
Mercury is added to the battery at the rate of a teaspoonf ul per 
8 hours when crushing the average (5 pennyweights) ore. The 
mortars of the two sections of the mill are of different pat- 
tern, those of the St. Mungo being the most roomy. As a 
consequence the latter have a slightly less crushing capacity 
but save a larger proportion of gold inside the battery. There 
is a considerable variation in the percentage of gold saved 
inside the mortar, but on an average it is about equal to that 
saved outside. 

The wells are four in number. One of them is cast in the 
lip of the mortar box, and owing to the vibration to which it 
is subjected all the mercury which it contains is shaken out, 
and it therefore serves no useful purpose. Just above the 
plates there is another deep well. In addition there are two 
shallow ones, also holding mercury. No blanket strakes are 
in use. 

The concentration of the pyrites in the ore is done by ma- 
chines of local manufacture and known as Halley's percus- 
sion tables. They are regulated at a speed of 171 to 175 
strokes per minute. The value saved in the concentrates 
represent 8 per cent, of the total yield from the ore. The 
amalgam retorts over one-half, about 54 per cent. The mill- 
stuff treated is very low grade, but owing to the width of the 
ore bodies the Lady Barkly Company has paid good dividends 
on 5 pennyweights material. 

The Pearl mill belongs to the mining company of the same 
name. It was erected comparatively recently, viz, 1888. 
The plant consists of six batteries containing five heads each. 
Each stamp weighs 7^ hundredweights. The number of drops 
per minute varies from 72 to 76. The height of the drop is 
kept at from 7 to 7^ inches. The issue or depth of discharge 
has the variation noticed at most of the mills. When start- 
ing with new dies the depth is 2 inches, attaining a maximum 
of 4^ just before they are removed. The dies are 3^ inches 



GOLD MILLING AT BBNDIGO, VICTORIA. 139 

"thick (occasionally 4 inches) ; they weigh 80 pounds each and 
are made of wrought iron. The shoes are made of cast iron ; 
they have a depth of 9 inches and weigh 192 pounds each. 
The wesLT per ton of ore is at the rate of 19 ounces for the 
shoes and 4.7 ounces for the die. 

The crushing capacity of the mill is at the rate of 400 tons 
per week of six working days. The mine is not, however, 
often able to supply the full amount of ore required. 

The gratings are of the usual round-punched iron plate. 
The kind generally in use contains 168 holes per square inch. 
When working full time the gratings have an average life of 
10 days. 

The gold saving is effected by the mortar box, by amalga- 
mating tables outside, by wells, by blanket strakes, and finally 
and indirectly by concentrating machines. Mercury is added 
to the ore when in the battery. Of the total gold obtained 
from the treatment 8 per cent, is the yield from the blanket- 
ings and concentrates, 7 per cent, comes from the wells, 20 per 
<5ent. is extracted on the amalgamating tables, and 65 per 
<5ent. is obtained from inside the mortar box. 

A clean-up is made of the battery once per week, and of 
the copper plates outside once per fortnight. The amalgamat- 
ing tables are covered with plain copper; they are 5 feet 
wide and 11 feet long, and they have a gradient of 1| inches 
per foot. The blanket strakes are subdivided into three 
longitudinal partitions, the total width being 5^ feet, having 
a length of 11^ feet. They have a slope of 1^ inches per 
foot. The residue from the washing of the blankets under- 
goes separation on straight sluices or " tyes," whence the sand 
obtained goes to the concentrators, and the pyrites is added 
to the concentrates from the shaking tables. 

These last are of the ordinary variety (Halley) and are six 
in number. They are worked at a speed giving them 166 to 
180 strokes per minute. 

The order of the drop of the stamps is 5, 1, 3, 4, 2, but 5 and 
1, 4 and 2 fall almost simultaneously. It will have been 
noticed that in spite of a very shallow discharge the largest 
portion of the yield is saved by the mortar itself. This, as is 
suggested by the high percentage of bullion obtained in 
retorting, is owing to the coarse character of the gold in the 



140 STAMP MILLING OP GOLD ORES. 

ore. From the grating to the nearest point of the shoe the 
distance is 3 inches, between the dies } inch, from the back 
of the mortar to the nearest point of the die 2 inches, and from 
the die to the front of the mortar, 1 inch. 

The feeding ,of the battery is done by manual labor. In 
order to keep them supplied the 30 stamps require the ser- 
vices of two laborers per 8-hour shift, one of whom, a young 
fellow, gets 33s. ($7.92) per week, while the other, an older 
man, is paid 40s. ($9.60). 

The country rock in which the quartz lodes occur forms a 
large proportion of the millstuff. It has a very variable 
hardness, from soft slate to quartzitic sandstone. An effort i& 
made to mix the material so as to assist the regular working 
of the mill. 

The New Chum Consolidated mill is the best in Bendigo. 
It was erected in 1889 and is in excellent working order. The 
motive power is obtained from a triple-expansion engine sup- 
plied with steam by two multitubular return flue boilers.* 

The plant consists of 30 stamps, arranged in two sections of 
three mortars. Each stamp falls with a weight of 8 hundred- 
weights. The boss or tophead weighs 193 pounds, the shoe 
175 to 185 pounds. The speed varies from 70 to 75 drops per 
minute. The height of the drop is rarely changed from 9 
inches. The depth of discharge is 2 inches at the time of 
placing new dies in position, and increases to a maximum of 
5 inches as they wear down. The dies are 4 inches thick. 
The crushing capacity of the mill is at the rate of 725 tons per 
fortnight of 12 working days. The gratings or screens are 
made of round-punched Russia iron, containing 170 holes per 
square inch. The holes are 0.023 inch in diameter. The 
gratings last about 17 full working days. They would give 
better wear but for the vertical lines of weakness produced 
by the press employed in their manufacture. 

The gold saving is done by the mortar box itself, by wells 
or mercury traps outside, by amalgamated tables, and indi- 
rectly by concentrators and blanket strakes. The mortar 
becomes an amalgamator by the addition, at regular intervals, 

♦This may appear hardly worth mentioning, but the fact Is that boilers of 
modern design are rare in the colonial mining centers. The single flue Cornish 
boiler forms the prevailing type. 



GOLD MILLING AT BBNDIGO, VICTORIA. 141 

of free mercury to the ore, as it is fed into the battery. A 
teaspoonful — about 2^ ounces — is added to each five stamps 
€very shift of ei^ht hours. The distance from the screen to 
the shoe is 3^ inches. As a rule in this district it is from 4 
to 5 inches. In the most recently erected plants there has 
been a tendency to make the mortar more roomy in order to 
increase the proportion of gold saved inside. 

The amalgamating tables are lined with plain copper plate. 
Their length is 10 feet, their width 5^ feet, and their slope 1^ 
inches per foot. The length is subdivided by a " well " or 
deep mercury trap, and three '' ripples" or shallow catch-pits. 
The first piece of plate is 23 inches long, and its surface is 
broken at a distance of 8 inches from the mortar by the deep 
well above mentioned. It is 7 inches deep and is always kept 
full of mercury. It holds the contents of one flask, or 75 
pounds avoirdupois. The ripples are distributed over the 
remainder of the length of the tables. 

From the "ripple tables" — as the Bendigo millman often 
<5alls the amalgamated plates — the pulp passes not first over 
blankets and then to concentrators, but vice versa. The con- 
centrators are Halley percussion tables worked at a speed of 180 
to 200 strokes per minute. There are six of them or one to each 
mortar. Then come the blanket strakes, which have a total 
length of 14 feet 9 inches and a width of 5 feet 6 inches. The 
width is subdivided into three partitions. The slope is equal 
to a fall of 1^ inches per linear foot. The blanket residues 
are usually very poor, and consequently the blankets are only 
washed at intervals of eight hours. A test lot is occasionally 
«ent to the chlorination works so as to determine what kind 
of work the concentrators are doing. A recent lot, resulting 
from a fortnight's crushing, amounted to only 4 tons, yielding 
6 ounces of gold altogether. When found to be poor, as in 
this instance, it is th« usual custom to return the blanket sand 
for retreatment by the concentrating machines. 

The four consecutive fortnightly mill runs, shown on next 
page, will give an idea of the kind of work the mill does. 

Thus 2900 tons were crushed for a production of 459 ozs. 8 
dwts. in retorted gold obtained in the mill itself, and 95 ozs. 
15 dwts. in melted gold extracted at the chlorination works. 
The ore, therefore, gave a total yield of 3 dwts. 4 grs. per ton. 



142 



STAMP MILLING OF GOLD ORES. 



Each "load" of concentrates is equal to about 27^ hundred- 
weights, therefore the yield per ton was 1 oz. 16 dwts. The 
concentrates amounted to 1.8 per cent, of the weight of the 
original ore. The gold from the pyrites is .970 to .975 fine, 
while that obtained at the mill is from .950 to .960 fine. In 
retorting the amalgam yields 46 per cent, of its weight in 
gold. Of the total amount of amalgam obtained, 55 to 60 
per cent, comes from inside the mortar box, and the re- 
mainder from the plates and wells outside. Of the amount 
saved outside the battery more than half is obtained from 
the deep mercury well. Of the total saving of gold affected 
by the mill, 81 per cent, comes by amalgamation and 19 per 
cent, by concentration. 





Yield by amalgamation. 


Yield by concentration. 


Ore. 


Bullion. 


Loads of 
concentrates. 


Yield in gold. 




Ozs. 


Dwts. 


Ozs. 


Dwts. 


737*ton8, 
710 " 
750 " 
708 " 


102 
100 
107 
149 


15 
5 
2 
6 


10 
9 
9 
9 


22 
28 
19 
25 


12 
5 
5 

18 



The loss of mercury is at the rate of one bottle (75 pounds) 
per 6 to 7 weeks. This is equal to a consumption of 7} penny- 
weights per ton of ore crushed. About 150 ounces of mercury 
are in use for each battery, with its series of wells and plates^ 

The following additional particulars will be of interest: 
The feeding of the ore is done by hand, and done badly. 
This will be referred to again. The shoes weigh 175 to 185 
pounds ; they last 9 to 10 weeks, crushing 115 tons of ore ; 
when worn out they weigh 38 pounds. The wear is therefore 
at the rate of 19.7 ounces of iron per ton of ore. The dies 
are 4 inches deep and 10 inches in diameter. They are square 
in section, with corners beveled. When new they weigh from 
96 to 100 pounds ; when discarded, 26 to 35 pounds ; they last 
28 weeks, crushing 335 tons. The wear is therefore at the 
rate of 3.4 ounces per ton of ore. The shoes are made of 
cast iron and the dies of vs^rought iron, provided by the local 
foundries at a uniform rate of £12 per ton. 

After a half year's operations the copper plates are over- 
hauled and cleaned with acid. At a Christmas clean-up this 



GOLD MILLING AT BENDIGO, VICTORLA. 143 

process yielded 646 ounces amalgam, giving 235 ozs. 4 
dwts. of gold from the treatment of the 18 copper plates. 
This cleaning spoils the gold-saving qualities of the plates, 
because its robs them of their surface of gold amalgam. It 
is done in order to squeeze out an extra dividend. 

There is not suflScient fall to carry away the tailings. They 
are pumped into a first or " slum" dam, where they are allowed 
to settle, then the clear water is pumped into a second dam, 
whence it is returned to the mill. 

The New Chum Consolidated Mining Company is one of the 
very best managed mining concerns in Australasia, as the fol- 
lowing figures testify : For the half year ending June 80, 
1890, there were 9586 tons sent up from the 1800-foot level, 
yielding 1722 ozs. 19 dwts. of gold at the mill, and 278 ozs. 
8 dwts. (from 106 loads) at the pyrites works. The total 
value of the yield was £7948 5s. 9d. The average of the ore 
was 3 dwts. 14 grs. only, but this gave a profit of £1510 6s, 9d., 
equivalent to a shilling dividend on each of the 28,000 shares 
of capital stock. The yield was 16s. 9d. (say, $4) per ton, and 
the cost 18s. 5d. (say, $3.22) per ton. 

The tailings of the mill have been carefully sampled and 
assayed. They are said to carry only 1 dwt. 4 grs. per ton. 
The right to treat them is sold to Chinamen for £8 5s. per 
month, one of the conditions being that the Chinamen shall 
maintain the dams in good condition. 

The five mills which we have passed in review are fairly 
representative of the best practice of the district. The Cath- 
erine Keef United is one of the oldest crushing plants, the 
New Chum Consolidated is one of the most recently erected, 
while the other three belong to intermediate periods. 

At Bendigo, as in the most of the Australian mining 
regions, the mines and mills are closed down on Sunday, and 
the week, therefore, has only six working days. This pleasant 
custom does not seriously interfere with the work in the 
mines, for they are comparatively dry ; nor at the mills, for 
the supply of ore does not require the use of the seventh day. 
It will have been noticed in the course of the detailed descrip- 
tion of the several mills that many of them do not work con- 
tinuously throughout the 24 hours. The majority of the mills, 
except in periods of unusual activity, as during the past 18 



144 STAMP MILLING OF GOLD ORES. 

months, work two-thirds time, or 16 hours. That this state 
of things should obtain is due to the desire, easily understood, 
of each mining company to possess its own mill and to treat 
its own output. The frequent fluctuation in the magnitude 
of the ore production causes the necessity for this intermit- 
tent operation of the mills, which are themselves, as a rule, 
proportioned in their capacity to an output in excess of the 
average supply that the mines can afford. The production of 
most of the mines is spasmodic, due to the fact that their 
development is ill-regulated and their ore reserves, at any 
given time, very small. No serious attempt is made to main- 
tain a uniformity in the output ; therefore a mine which is 
producing a full mill supply for a certain period may suddenly 
become non-productive until fresh development work leads 
to the uncovering of new bodies of ore. 

This is a mistake, of course, not only in respect of general 
management, but also in regard to the mill itself. A mill can 
be run at less expense per ton of ore when it is kept continu- 
ously at work than when working at intervals. Periods of 
idleness must be injurious to the machinery, and not only to 
the delicate mechanism of the engine, but also to the station- 
ary parts of the plant. Rust and dirt find a ready entrance 
everywhere when a mill is working 16 hours and idle the 
remainder of the 24. 

The comparatively small size of the mills calls for comment. 
The average number of stamps is 30. In one case — the Lady 
Barkly and South St. Mungo — two mining companies share 
one mill. This is a step in the right direction. If there were 
fewer mills, but larger ones, if the number of stamps now 
working in two or more plants were consolidated under one 
roof, there would result an economy in labor, power, and 
superintendence which would help very considerably to de- 
crease the cost of milling per ton of ore. 

The mills are well-built, neat machines. They are all of 
local manufacture, and reflect much credit upon the Bendigo 
foundries. Their framework is entirely of iron. (See 
accompanying drawings.)* This adds much to their appear- 
ance, but it has often been questioned whether it does not 
also increase the vibration produced by the fall of the 

* Wliloh 1 owe to the courtesy of the manager of the Victoria Foundry, Bendigo. 



GOLD MILLING AT BENDIGO, VICTORIA. 145 

stamps. It is generally supposed that a wooden frame is less 
rigid, and therefore takes up the vibration better than iron, 
which, on the contrary, is supposed to become crystalline and 
therefore brittle. The effect of the vibration upon the 
structure of the iron is a point which has been matter for 
controversy. This is not the time to discuss so difficult a 
subject. The experience of the Australian mills does not 
sustain the idea that an iron framework is objectionable. 
The iron of the stamp stems becomes crystalline and breaks 
do occasionally occur ; so also with the cams, but not more 
often than in the case of wooden-framed batteries. The 
Garden Gully United mill, for instance, which was one of the 
first iron-framed plants erected in this district, is not fully 20 
years old, but is still working very smoothly. 

Before entering into a discussion of the character of the 
work done and the milling methods employed, it will be neces- 
sary to look at the nature of the ores treated. The Bendigo 
reefs are in slate and sandstone country. The millstuff con- 
tains a very large proportion, varying from 25 to 75 per cent., of 
" mullock " or country rock. On the whole, I think the sand- 
stone predominates over the slate, but the ratio is constantly 
changing. The sandstone is, of course, the harder of the two, 
but it breaks in a granular way, which is more favorable to 
amalgamation than the tendency toward the formation of 
slimes which characterizes the crushing of the slate. In 
many of the mills an effort is made to mix the two. 

The gold exists in a quartz more white than that of the 
free-milling ores of the Californian foothills. The amount of 
sulphides occurring in the ore is small but very variable. 
Upon an average the millstuff contains less than 2 per cent, 
of pyrites. The other sulphides — blende and galena — are 
present in insignificant proportion. The quartz itself breaks 
readily ; it often has a sherd-like fracture, causing it to splinter 
easily. The gold is essentially coarse ; the retort percentage 
at the mills (rather over than under 50 per cent.) is indicative 
of this fact. The gold is also of high caratage, as shown by 
the bullion obtained at the mills, this being rarely under .950 
fine. The gold as it exists in the ore is not rusty, but bright 
and in a condition to be readily and quickly amalgamated. 



146 STAMP MILLING OP GOLD ORES. 

The silver which accompanies it is in such minute quantity 
as to be negligible. 

The millstuff crushed and treated by the Bendigo mills is 
therefore essentially free milling. The extraction is, as a 
rule, high. As to the values which escape in the tailings, I 
estimate them at from 1^ to 2 pennyweights. 

Mr. J. Cosmo Newberry, the government analyst, states 
that " the average tailings of the quarz-mining districts of 
the Colony can be considered as containing between 2 penny- 
weights to 2i pennyweights per ton, while in some dis- 
tricts, as, for instance, Sandhurst (or Bendigo), where the 
methods of treatment are good, the loss, on the average, is not 
more than 1^ pennyweights per ton." The mean average 
of the ore treated at the mills has, for a period of five years, 
varied from 9 dwts. 2 grs. to 9 dwts. 23 grs., so that a loss of 
1^ pennyweights would represent an extraction of 85^ per 
cent., a result which is decidedly good when compared to the 
work done in other mining regions. Though good in com- 
parison, it yet easily falls short of what it might be. The 
mere fact that Chinamen can find it profitable to treat the 
tailings discharged by the mills indicates that the gold is in 
such a condition as to render its loss avoidable if the proper 
methods were employed. The loss is due to the irregular 
pulverization caused by bad and uneven breaking and feeding 
of the ore. This I hope to prove. 

In considering the milling practice it will be well to distin- 
guish between the actual operation which extracts the gold 
and the disposition of the parts of the machine which does 
the work — that is, between what is chemical and metal- 
lurgical, and what is essentially mechanical. The former may 
not admit of adverse criticism, and the treatment may be 
considered technically correct, but the latter falls far short of 
attaining any perfection of automatic work and is decidedly 
bad. 

At all the mills a large part, nearly one-half, of the total 
amount of amalgam obtained is derived from the saving 
effected by the mortar box itself. There are no inside plates, 
the gold being arrested by the action of gravity, assisted by 
the agency of the free mercury added from time to time to 
the ore in the battery. The tendency in this locality at the 



GOLD MILLING AT BENDIGO, VICTORIA. 147 

time of my last visit was to augment the proportion of gold 
to be saved within the mortar box by making the latt.er more 
roomy. The distance from the screen or grating to the nearest 
point of the shoe is about 5 inches, and the latest designs 
indicate the intention to increase this figure. It is a fault in 
the ways of these old-established Colonial mining centers that, 
owing to the fact that the mining and milling are not under 
the direction of technically trained engineers, the mode of 
construction of the mills has necessarily to be left almost 
entirely in the hands of the foundries. The miilmen attend 
to the extraction of the gold in the ore, which it is their busi- 
ness to crush, but they do not concern themselves with the 
details of the design of the machine which does the work. 
The foundries, very naturally, change their patterns as little 
as possible, and there seems to be an unexpressed idea that 
the ores must adapt themselves to the mills rather than the 
reverse. In this respect Bendigo is not to be considered 
unique. The same thing is done elsewhere. 

In general, the methods of milling show a lack of progres- 
siveness and an absence of that acquaintance with, and 
immediate utilization of, modern improvements which char- 
acterizes the mining regions of the great West. That this is 
no unfair commentary is indicated in a very marked degree 
by the deficiencies of the mills in respect of the arrangements 
designed to promote an automatic handling of the ore. It 
may well be asked, Why do the mills, without exception,* 
use shaking tables which do not discharge their concentrates 
automatically, seeing that there are so many, and no more 
expensive, concentrators which will do this work without the 
aid of a boy and a shovel ? The reply to this query generally 
comes somewhat in the following form : That the boy is the 
deserving son of a deserving old father; that he does not 
receive high wages ; and that it is just as well to give work to 
such young fellows. The weakness of the logic does not 
manifest itself to the old Colonial, but, of course, the argument 
is quite untenable. The fewer boys and shovels you employ 
in your mill the more automatic as a machine it becomes, 

* I refer to the time of my stay at Bendigo. Since then Mr. George Lansell has 
introduced Gilpin County " bumpers," or shaking tables, at the Fortuna mill. 
They are working very satisfactorily. 



148 STAMP MILLING OF GOLD ORES. 

and the less will be the cost of treating the ore, the bigger 
will be the dividends, the more numerous will be the mines 
and mills that w^'U be operated, and the greater the chances 
of employment for every one in the district. 

This is, however, a small matter compared to the absence 
in every mill of rock-breakers, grizzlies, and self-feeders. It 
is impossible not to feel that this is a disgrace to any modern 
mining district. But Bendigo is not a solitary sinner. The 
gold production of the great mining colony of Victoria, 
namely 60,000,000 ounces in 40 years, places it in the forefront 
of the mining regions of the world, and the results, magnifi- 
cent as they are, have been obtained by the work inaugurated 
and carried out by men who are second to none in energy and 
ability. Nevertheless, in the whole Colony there are to-day 
only 12 rock-breakers, and of these I know of at least four 
which are in mills that are idle. To me it is incomprehen- 
sible that there should be in this respect such a lack of keeping 
step with that modern progress which has been no more 
marked in any department of human activity than in that of 
the mining industry. 

While at Bendigo I had many serious discussions over the 
rock-breaker and self-feeder question; so many that I am 
afraid I was mistaken for the advance agent of a machinery 
firm. But, you say. Queen Anne is dead! It is surely a 
waste of time in the year 1896 to be discussing the advisability 
of employing rock-breakers and self-feeders in a gold-quartz 
mill. It may, however, do us good and be of service to our 
cousins in Australia if we again inquire into the question as 
if it had not been settled long ago. 

Let us first take the cost of breaking and feeding at two of 
the best of the Bendigo mills : 





Stamps. 


Number of 
feeders per 
shift. 


Wages per 
week. 


Cost per 
annum. 


New Chum Consolidated . 


80 


8 boys. 


£9 


£468 


Pearl 


.. ao 


1 boy and 1 man. 


£10 19s. 


£569 



Boys at £1 per week are put down as inexpensive extras, 
but in a year the wages paid to them make a sum greater than 
is required to purchase and erect a full equipment (it would 
be six in each of the above instances) of the most expensive 
(but also most effective) of automatic ore-feeding machines, 



GOLD MILUNG AT BBNOIGO. VICTORIA. 



149 




An Iron-framed Battery as Erected at Bendlgo. 



STAMP HILLINQ OF GOLD ORKB. 



and a rock-breaker of such a capacity as would break a Si- 
hours' supply of ore in 10 hours, leaving the pulley which 
drives it by day free to operate a dynamo to illuminate the 
mill by night. 




Details of Battery al Bendlgo. 



But the economy is not that of labor only ; it extends par- 
ticularly to wear and tear. Good hand-feeding is more of a 
day-dream than an actuality ; it is not found at Eendigo, for 
there the feeding is done badly and irregularly. I have often 
stood by watching the boys assisting with the blows of a 



GOLD MILLING AT BENDIGO, VICTOBIA. 151 

sledge hammer the entry into the battery of pieces of rock, 4 
to 6 inches big, wnich were too large for the feeding hole and 
which they were too lazy to break up previously. They are 
not to be blamed so much as the system which makes them 
possible. 

Uneven breaking and irregular feeding add immensely to 
the wear and tear of the whole mechanism of the mill, A 
stamp which one moment is falling on soft pulverized par- 
ticles of ore is at another instant dancing upon a hard, large 
piece of rock, upon which it descends more than once before 
breaking it. If the stamp falls full upon it no work may be 
done, and that is all, save for the jar to the whole machinery ; 
but if it strikes it on the edge a piece of the shoe is in all 
likelihood chipped off. This can be proved by the actual 
wear of the shoes, as expressed in ounces of iron abraded per 
ton of ore crushed. The die is not affected to anything like 
an equal degree, because the layer of ore upon it acts as a 
cushion and tends to equalize the wear. Careful inquiry 
shows that while at the New Chum Consolidated and Pearl 
mills there are 19.6 and 19.4 ounces, respectively, of the shoe 
worn away for every ton of ore crushed, at Grass Valley 
(California), the Black Hills (South Dakota), and Mammoth 
(Arizona), the wear in no case exceeds 7^ ounces per ton of 
ore. In spite of the variety of castings, of the varying hard- 
ness of ore, and of other changing conditions, the evidence is 
wonderfully harmonious in accentuating the fact that the use 
of proper rock-breaking and ore-feeding machines very much 
decreases the wear and tear of shoes and dies. The time lost 
in a year by the more frequent stoppage of the work of the 
stamps in order to replace worn-out shoes with new ones, is 
by no means inconsiderable and means a monetary loss which 
must be added to that caused by the excessive waste of the 
metal of the shoes. 

The time of service of the gratings or screens is also seri- 
ously diminished by the bad feeding and rough breaking of 
the ore. So long as man is human there will come times when 
the feeder, taking a rest or wanting a smoke, will fill the 
mortar box with ore, so as to keep the stamp supplied during 
the interval which he requires. High feeding is a potent 
cause in diminishing the life of the screen. Facts and figures 



162 STAMP MILLING OF GOLD ORES. 

bear out this line of argument. At Bendigo a grating lives 
to pass through 110 tons of ore ; but in the principal milling 
centers of America, all of them using improved rock-break- 
ing and ore-feeding appliances, the screens, under ordinary 
conditions, last during the discharge through them of from 
200 up to 300 tons. The maximum figure is reached at mills 
which crush three tons of ore per stamp, but even at Grass 
Valley, where the millstuff is certainly very much harder 
than that of Bendigo, but where the rate of crushing is about 
the same, though the screens used are of greater fineness, 
they give an average service nearly twice as good as that ob- 
tained at the Pearl or New Chum Consolidated mills. 

More important, however, than the wear and tear of screens, 
shoes and dies, and other working parts of the mill, is the 
effect produced upon the crushing capacity of the stamps. 
By using rock-breakers, grizzlies, and feeders in place of the 
primitive barbarism of sledge-hammer, boy, and shovel, you 
save the stamp that breaking of the larger stones of ore 
which is at present its hardest and most wasteful work. The 
Bendigo mills would, I believe, judging from experience else- 
where, increase their crushing capacity 25 per cent, if properly 
equipped. This means nothing more nor less than a diminu- 
tion equivalent to 20 per cent, in the cost of milling. 

The stamp battery would also become a better amalgamat- 
ing machine. At present it is a breaker, pulverizer, and 
amalgamator, all combined. If the first and roughest work, 
that of breaking, were omitted and delegated to a machine 
especially designed and constructed to do this particular part 
of the operation both well and quickly, the stamps would 
pulverize more evenly and the mortar box would become a 
more efficient amalgamator. 

One could continue the discussion by referring to other 
parts of the mill, for regularity and evenness of working are 
as essential to the stamp battery as to any other machine, 
and the want of it is seriously felt in every portion of the 
contrivance which the miner uses to crush his ore and to ex- 
tract his gold. But enough has been said. This feature of 
the mills would not perhaps have been worthy of such length- 
ened consideration were it not that Bendigo is the greatest 
^tamp-milling center of Australasia, having gold mines which, 



GOLD MILLING AT BENDIGO, VICTORIA. 163 

for the Extent of their development in depth, are without a 
parallel and which promise a future of long-continued pro- 
ductiveness. 

In conclusion, while it may be said that the milling prac- 
tice of Bendigo is intelligent in its methods and successful in 
the extraction of a large percentage of the value in the ore, 
it must also be added that owing to the absence of certain 
modem improvements of unquestioned usefulness the results 
obtained, while good, are yet got at a quite unnecessary waste 
of labor and material. 



CHAPTER X. 
Double-Discharge Mortars in Victoria* 

The Ovens is one of the main tributaries of the Murray, the 
only great Australian river. Its source is in the Australian 
Alps, and its headwaters flow through the most rugged and 
picturesque portion of the colony of Victoria. This district 
is northeast from Melbourne, and near the border dividing 
Victoria from its neighbor. New South Wales. In the early 
fifties the mining camps of Bendigo and Ballarat sent forth 
bands of pioneers who penetrated the hearts of the snow- 
capped ranges and found placers of extraordinary richness, 
which made the Woolshed, the Buckland, and other localities 
in the Ovens district, rank among the very best discoveries of 
that golden age. Now, however, the former glory has 
departed, the Mongolian has come to pick up the crumbs 
which have fallen from the table of the Caucasian, and the 
more steady routine of quartz-mining communities has 
replaced the feverish excitement of alluvial diggings. The 
mines are scattered over a country the most mountainous in 
Australia, and the mills stand beside streams whose perennial 
flow is in pleasing contrast to the dusty dryness which is 
characteristic of most of the mining centers of Queensland, 
New South Wales, and Victoria. 

The methods of milling have been derived for the most part 
from Clunes. The mills themselves are usually small and do 
an irregular custom work, dependent upon a precarious and 
uncertain ore supply obtained from the small mines whose 
white waste-heaps dot the blue of the forests of eucalyptus 
(" the bush") which cover the surrounding hills. 

(164) 



DOUBLB-DISCHARGB MORTARS IN VICTORIA. 



155 



In the accompanying tabulated statement I have given the 
chief figures illustrative of the methods in use at five mills, 
of which only the first does not do custom work. 



COMPARATIVE TABLE. 



Ncme of mUl. 



Harrietville . 
*0riental . . . 
*HUl8borough 
*Bailway . . . 
^Stephens . . . 



Name of mUl. 



« 

I 

00 



O 



26 
16 

8 
20 

6 



s 



o 



Lbs. 



700 
784 
784 
720 
840 



Harrietville . 
^Oriental . . . 
*Hill8boroagb 
^Railway . . . 
*BtepbenB . . . 



«M<JJ 

d $ 

o S 
U O 
9 O 



u 
a> 

Pi 
09 
P. 

o • 

l2 

o P 

1^ 



Per cent. 



1 

2 

*• 

** 



70 
66 
60 
60 
60 



o 



In. 



8 

9 

9% 



9% 



I 

OB 

^ • 

4A OS 

p 



In. 



2 
8 
8 
4 

4J^ 



^4 

P4 

si 

eSfd 



Tons. 



I? 



o 

s 



Tons. 



87+ 
20 
14 
82 
9 



o 

d 

« bo 

P 



Bound- 
punched 
Russia 
iron. 



^ 




1 


i 


t£ 


OD 




d 


d 


dfe 


a 


Wi 


S 


(A 


^i 


9 


o 


u 


S 


« o 


4^ 

d 


Sd 


I 


bo 


Sd 


old CO 
same. 


Inene 
bullio 


etort 
age. 


5 


OSS of 
per tc 


O 


fo 


« 


H? 


Ozs. 


Per 1000. 


Per cent. 


Days. 


Dwts. 


6 


966 


86 


18 


19t 


** 


940 


62 


17 


8 


1^ 


940 


SO 


18 


4 


** 


960 


46 


20 


8 


** 


946 


48 


18 


8 



4a 



5>«jd 
o®d 

ddS 



240 
220 
200 
200 
260 



d 
o 

•rH 

4d 

A 

a 

o 






Gals.$ 

6 
4 

8M 
4 

4}^ 



The Harrietville mill is much the most important in the 
district. It is the property of an English company owning a 
very extensive group of mines, among which may be men- 
tioned such euphonious names as " Tiddle-Dee- Addle-Dee," 
''Jackass," " Monsmeg," etc. 

The plant consists of 25 heads, in two sections of 10 and 15 
stamps respectively, separated by the overshot waterwheel 
which supplies the motive power. The wheel has a diameter 
of 40 feet and a breast of 5 feet. 

Each stamp weighs 700 pounds and drops 70 times per 
minute. The height of the drop averages 8 inches. When 



* Custom mill. 

f Long tons, 2240 pounds. 

X Including pans. 

$ Per stamp per minute. 

*♦ Not saved. 



166 



6TAHF MILLINQ OF GOLD OBES. 



new dies have just been placed in position, the depth of dis- 
charge (the distance from the bottom of the grating or ecreen 
to the top of the die or false bottom) varies from one-half to 
one inch, depending upon the amount of sand packed under- 
neath the dies. On measuring the depth of discharge in 
several batteries at the close of a month's working, I found it 
to be 2i, 3, 1, 1^, 1, 2i, 1|, or an average of If inches. 

The entire mill treats 1100 tons of 2240 pounds per month. 
Each stamp crushes 1^ tons per 24 hours. 




DaDble-dlschai^ Mortar Used at The Har 



The coffer or mortar box is 5 inches deep, while the dies, 
which are octagonal, are 4 Inches thick. The shoes hav& 
round tongues, and are 9i inches in diameter by 9 inches high. 
A shoe weighs 172 pounds, and a die 84 pounds. Both are 
made of best quality white iron, fagoted, not cast. This costs 
16 shillings per hundredweight (112pounds). The shoes wear 
evenly, but the dies soon develop an irregular surface 
("cupping") and exhibit much variation in their time of ser- 



DOUBLB-DISCHARQE MORTARS IN VICTORIA. 157 

vice. The average wear of iron per ton of ore crushed is at 
the rate of 9.8 ounces of the shoe and 3.4 ounces of the die. 
The ore being of comparative softness, the wear of the shoe 
must be considered as excessive. This result is due to two 
causes, the non-employment of a rock-breaker and the use of 
an identical material in both shoe and die. By the more 
uniform breaking of the millstuff the shoe can be saved much 
violent work, and by making the die of a metal less hard and 
more tough than the shoe, the latter would be found to last 
longer and to cause the die itself to retain a more even 
wearing surface. 

The coffer is of peculiar design, and is provided with an 
end. discharge, as is illustrated by the accompanying drawings, 
which I owe to the courtesy of the manager, Mr. Thos. G. 
Davey. The clear space inside the mortar is 4 feet 6 inches 
long by 1 foot wide. The interior is protected by a cast-iron 
lining 1 inch thick, and divided into four parts. These are 
shown in position in the section elevation. (See illustration. ) 
The bottom of the mortar is flush with the amalgamating 
table outside. A part of the front edge or lip of the mortar 
box is removable, and allows of the easy introduction or 
removal of the dies. 

The grating frame is in three sections, of which the two 
frames at the ends are tjurved, but afford a discharge surface 
similar in extent to that of the front, which is straight and 
has a length of 2 feet. The screen itself is of round-punched 
Kussia iron. That at the ends has 175 holes per square inch, 
while that in front has 240. The time of service is slightly in 
favor of the front grating. A set lasts from under three 
weeks to nearly a month — that is, during the crushing of from 
125 to 200 tons of ore. In this mill, as in many others which 
came under my notice, complaint is made of the weakening 
of the gratings by the perpendicular lines formed by the 
action of the press used at the time of their manufacture. 
The gratings are not fixed to a frame, but are kept in place 
by an iron clamp. 

The gold saving is done in the mortar box itself, by outside 
amalgamating tables, by wells, and indirectly by shaking 
tables, the concentrates from which are first roasted and then 
treated by amalgamation in pans. 



168 STAMP MILLINa OF GOLD ORES. 

There are no amalgamating plates inside the mortar. Mer- 
cury is added to the ore fed into the battery, in quantities 
varying with the richness of the millstufF. The average 
amount is about one ounce of mercury per battery (of five 
heads) every half hour. The regulating of this addition is 
determined by the examination of the plate which forms the 
head of the amalgamating table. Every four hours this is 
inspected. If it be "moist," less mercury is added to the ore 
in the battery ; if it be *' dry," more is introduced. 

The amalgamating tables are 12 feet long and 6 feet wide. 
They have a slope of seven-eighths inch per foot.* The total 
length is distributed over four subdivisions, of which the two 
uppermost are lined with silver-plated copper and the two 
latter with plain copper. The first or apron-plate is 22 inches 
long, and the second 3 feet. 

A variation in the arrangement of the platee of the tables 
of two adjoining batteries enabled an interesting comparison 
to be made. At No. 5 battery the succession of plates is 
silver-plated copper, plain copper, plain copper. The middle 
one of these came from the old Monsmeg mill, was thoroughly 
amalgamated, and in first-class condition. At No. 4 battery 
the order was silver-plated copper, silver-plated copper, plain 
copper. After a year's working with ore of identical char- 
acter it is found that the plain copper forming the third plate 
of No. 5 battery is well amalgamated, while the corresponding 
plate of the same material at No. 4 has not yet become 
whitened by amalgam. This proves that the second plate at 
No. 5 — viz, plain copper — is not arresting gold and amalgam 
so successfully as the corresponding plate — viz, silver-plated 
copper — at No. 4 battery. 

The amalgam is cleaned up every three days. Immediately 
following the first length of plate there is a drop-well 3^ inches 
deep. There is also a shallow well or ripple following this 
drop-well and succeeding each of the other subdivisions of 
the tables. All are charged with mercury and cleaned up 
once a week. Three bottles are in use for each set of tables. 
Thus 15 bottles are in use for the entire mill. The loss of 
mercury amounts to about 75 pounds avoirdupois per 1100 

* Sucb a sligbt inclination of the plates is rendered practicable by the large 
ainotint oi water ased in fhtf: mill. 



DOUBLB-DISCHARGB MORTARS IN VICTORIA. 159 

tons of ore crushed, being at the rate of 19 pennyweights troy 
per ton. This includes that consumed in the pan amalgama- 
tion of the concentrates. In the mill proper the consumption 
would be about 8 pennyweights. 

There are no blankets below the tables, the pulp passing on 
at once to the shaking tables. There are five of these. They 
are the usual colonial variety of the Rittinger percussion 
table, and are not nearly so effective as some others designed 
on the same principle, as, for instance, the "bumper" of 
Colorado. The speed is regulated at 135 strokes per minute. 
The distributor of each concentrating machine is lined with 
old copper plates which are intended to arrest floured quick- 
silver and escaping amalgam. The ore yields about 1 per 
cent, of concentrates, consisting mainly of iron pyrites. The 
concentrates contain 5 ounces of gold per ton, or at the rate 
of 1 pennyweight per ton of ore crushed by the mill. 

The general clean-up takes place on the last day of each 
month, and requires six hours for its completion. The dies 
are taken out and any adhering amalgam is scraped off. The 
sand inside the mortar is collected and washed over the amal- 
gamating tables. This serves to catch most of the amal- 
gam. The sand passes on to the shaking tables, whence the 
heaviest portion is returned to the battery when the mill is 
restarted. Of the amalgam obtained, 33 per cent, is collected 
inside the mortar boxes ; of the remaining two-thirds, 8 per 
cent, comes from the wells and 58 per cent, from the amalga- 
mating tables. Of what is saved on the plates fully 90 per 
cent, is yielded by the first length of the plate. Of the total 
yield of gold 86 per cent, is extracted by amalgamation and 
from 7 to 12 per cent, is saved in the concentrates. The 
retort yield of the amalgam varies from 25 to 52 per cent., 
averaging 36 per cent. The bullion is of high caratage, 
equivalent to .965 fine. Upon measurement it is found that 
the consumption of water is at the rate of 5 gallons per stamp f^^ 
per minute. This comparatively large amount is necessitated 
by the clayey character of the ore, the use of the end dis- 
charge, and the flat grade of the amalgamating tables. 

The framework of the mill consists of iron standards of a 
comparatively light pattern. Owing to the care taken in the 
building of the foundations there is, however, no excessive 



^^1 



'\j 

K 



160 STAMP MILLING OP GOLD ORES. 

vibration. The tappets or discs are kept in place by a key 
and not by the old discarded, but more common, Colonial 
device of a screw. It was found here as elsewhere that while 
screw tappets are very excellent in theory, and while they 
may answer admirably when first put in position, as soon as 
they become the slightest bit loose, which cannot be prevented 
eventually, the screw gets instantly worn and is soon ruined. 
This necessitates too frequent stoppage for the cutting of a 
fresh thread. 

The order of the drop of the stamps varies in the two 
sections. Fifteen heads drop in the order of 5, 1, 3, 4, 2, while 
ten fall in pairs, 5 and 1, 4 and 2, 3. The latter style is, 
unfortunately, too common in the Colonies. I noted that the 
stamp made a complete turn in from four to seven drops. 

This is an excellent mill excellently managed ; but it has 
one serious defect, a defect only too common in Australian 
mills. I refer to the absence of a rock-breaker. The Harriet- 
ville plant is in advance of many of the Colonial mills in being 
equipped with self-feeders, which, while simple, perhaps 
crude, in design, are yet effective in practice. For the absence 
of a rock-breaker there can be no excuse. The ore, it is true, 
is of less than ordinary hardness ; most of it is small when 
broken in the stopes of the mines, but there is a proportion 
of larger stones sufficient to prevent anything like uniformity 
of size and to cause an excessive wear of the iron of the 
stamp shoes. For breaking ore a machine constructed upon 
the principle of a multiplied lever must necessarily be better 
adapted than one designed on the idea of a falling weight ; 
hence the rock-breaker is better qualified to break ore than 
is the stamp. (Pulverization is another question.) By com- 
pelling the latter to do work which should be previously done 
by the former you produce unnecessary waste of the material 
of the shoes, dies, screens, and other wearing parts; you 
seriously diminish the crushing capacity of the mill and also 
interfere with the regular operation of the amalgamation 
which saves the gold. 

We have seen that the mortars in use at this mill are 
designed so as to have a discharge at the two ends, as well as 
in front. It will be remembered that at Clunes the batteries 
have both a back and front issue. These variations from a 



162 STAMP MILLING OF GOLD OBES. 

dull uniformity are interesting. At the Harrietville mill the 
end discharge is certainly successful. If it does nothing more 
it enables the use of copper tables of an unusual width, and 
by so doing it increases the area of amalgamating surface 
and the thin distribution of the pulp. An amalgamating 
table 6 feet wide and 12 feet long is a sight to gladden a mill- 
man's heart. As observed by me the wash of the pulp over 
the tables was very regular in speed and even in distribution. 
The Colonial millman will generally hasten to condemn the 
use of an end discharge. The reason is not far to seek. It is 
ordinarily attempted under the very unfavorable conditions 
afforded by a mortar box of rectangular shape. The result is 
that the issue through the end gratings is both weak and 
irregular, while at the same time the discharge from the 
front is injuriously affected. 

At Harrietville these difficulties are overcome by a change 
in the design of the mortar box itself, and by using screens of 
different mesh. The two ends and the front of the mortar 
make a curve which is an arc of which the straight line of the 
back of the mortar is the chord. The discharge surface is 
subdivided into three equal portions, each 2 feet long. The 
splash at the ends being weaker than in front the former have 
gratings pierced with 175 holes per square inch, while the 
latter is provided with a grating carrying 240 holes. 

While the mortars used at Harrietville are well designed, 
nevertheless the writer does not wish to be understood as 
recommending the double-discharge mortar. The ore of the 
mine is easy to crush and the gold which it carries is readily 
amalgamated, therefore he is of the opinion that if the mill 
were equipped with rock-breakers and self-feeders of approved 
design, and if the mortars were of the simple single-front 
discharge kind, if the stamps were a little heavier and fell a 
little faster, that then the capacity of the mill would be very 
considerably increased without a decrease in the percentage of 
gold extracted out of the ore. 

The concentrates from the percussion tables are roasted and 
amalgaLiated. The roaster is a simple reverberatory furnace. 
The hearth is 27 feet long and 9 feet wide. Its length is 
subdivided into three equal portions, each separated by a 
drop of 2 inches. The charge is 12 hundredweights. The 



• to 



DOUBLE-DISCHARGE MORTARS IN VICTORIA. 168 

time taken is eight hours. The daily capacity of the furnace 
is four tons. Immediately below the roaster is the amalga- 
mation plant, consisting of two Wheeler pans and one settler. 
The extraction of the gold is exceedingly good, ranging from 
90 to 97 per cent, of the assay value of the concentrates. 

The Harrietville mill is distinguished among Australian 
mills in that the work done is checked by regular and system- 
atic assays. A sample of the tailings is taken every hour, the 
resulting pulp being assayed each day. This is a feature of 
the methods of management worthy of unqualified praise ; it 
is a portion of mill work only too rarely attended to, with the 
consequent loss of gold due to bad milling which is the direct 
result of the ignorance of the millman as to what he is really 
doing. 

The extraction of the gold in the ores treated by this mill 
is particularly good, the tailings containing often as low as 
five grains per ton. In the actual results obtained and in the 
systematic methods employed, this plant reflects much credit 
upon the local English management. 

Referring to the comparative table given in the earlier por- 
tion of this work, it will be seen that the other four mills 
whose figures are given have a general similarity in their 
methods. It will not be necessary to describe more than one 
of them, and the' one chosen will be the Oriental mill, at 
Wandiligong, situated at the foot of the range which divides 
the Ovens River from one of its affluents. This mill is old 
and is a type of many of the small "crushing machines" 
which are scattered among the valleys of the Australian Alps. 
The two sections of the mill, each containing eight stamps, 
are separated by the overshot waterwheel, 27 feet in diameter, 
having a 3^ feet breast. Each stamp weighs 7 hundredweights. 
The ore is broken at the mines, any large lumps arriving at the 
mill being subjected to sledge-hammer treatment. There is ,^ , 
no rock-breaker. The ore-bins are, however, supplied with 
self-feeders of a simple but effective type. 

The tappet of the stamp strikes a rod which communi- 
cates the shock to the chute leading from the ore-bin. The 
hand screws enable the millman to increase or decrease the 
angle of the chute, according as the ore is coarse or fine, wet 
or dry. This feeder costs £8 and works well. 



L / 



•^ f 



164 STAMP MILLING OF GOLD ORBS. 

The stamps have a drop of 9 inches. When new dies have 
been placed in position the depth of discharge is 2 inches. 
The stamper box or mortar is 6 inches deep, while the die or 
false bottom is 4 inches deep. The maximum depth of dis- 
charge would be 4 to 4^ inches. The speed of the mill is reg- 
ulated at a rate of from 55 to 60 drops per minute. The 
crushing capacity is 120 tons per week of six working days. 
The grating or screen is of round-punched Russia iron, having 
220 holes per square inch. The pyrite in the ore is not saved, 
but in place of concentration the tailings are treated in 
Wheeler pans. The amalgam retorts 52 per cent., varying 
from 40 to 60 per cent. The bullion is worth £4: per ounce, 
equal to a fineness of .940. 

The gold saving is done by the mortar box, by wells, by 
amalgamating tables, and finally by. pans outside the mill 
itself. Mercury is added to the ore fed into the battery, and 
the mortar thereby becomes an amalgamating machine. With 
ore containing an ounce per ton a pellet of mercury of the 
size of a pea is added every two hours. 

The amalgamating tables are covered with plates of plain 
copper. They have a total length of 15 feet and are sub- 
divided into four sections, the first being 3 feet and the others 
4 feet each. At the bottom of the first length of plate, some- 
times called the "apron," there is a well 6 inches deep, while 
at the bottom of the next two divisions there is in each case 
a well 4 inches deep. Each plate is further subdivided by a 
shallow well or ripple which crosses it half way down. This 
is 3 inches wide and three-eighths inch deep. There are thus 
altogether three wells and four ripples. 

Of the total amount of amalgam saved, 2 per cent, is caught 
in the wells, most of it in the first one, 48 per cent, comes 
from the amalgamating tables, and the remainder from the 
inside of the mortar box. 

The tables have a fall of three-fourths inch per foot. This 
is slightly under the average of Australian mills and necessi- 
tates the use of an increased amount of water. The passage 
of the pulp over the amalgamated surface is slow, and this 
fact forms a good feature of the arrangement, but on the 
other hand a very nice adjustment of the supply of water 
is demanded. 



DOUBLE-DISCHARGE MORTARS IN VICTORIA. .165 

In certain cases, where customers so desire, a copper plate 
12 inches wide by 15 inches broad is attached to the " splash- 
board" immediately outside and facing the grating. It re- 
ceives the splash of the pulp as it issues from the battery, 
and is placed at such an angle that the pulp will just not pass 
over the upper edge, but take an up-and-down wave movement 
over its amalgamated surface. It is a very efficient aid in 
collecting the gold. 

As a rule only the first length — 3 feet— of copper is used on 
the tables. Frequently customers place plates of their own 
over the remaining length. 

The blanket strakes below the tables are 12 feet long, with 
a gradient of IJ inches per foot. They are not often brought 
into use. 

The tailings pass outside the main building and are con- 
ducted to a tailings mill, shortly to be described. Contrary to 
the usual Victorian practice wooden guides are in use. The 
wood obtainable in the immediate neighborhood is very suit- 
able for this purpose. The millman* informed me that a set 
of top guides made of red gum {eucalyptus rostrata) has lasted 
for 16 years. The lower set does not wear so well, since it is 
splashed by the water from the battery and injured by the 
accompanying grit. Iron guides are found to require ten 
times as much grease for their lubrication. 

The shoes and dies are both made of wrought iron. The 
latter wear down very evenly and can be used until very 
thin. The shoes do not give such good service on account of 
the softness of the material, which is not well adapted for the 
work to be done. 

The tailings plant consists of two grinding pans and one 
amalgamating pan, the motive power required being derived 
from a waterwheel 24 feet diameter and 2 feet breast, oper- 
ated by the tail-water of the upper millwheel. The grinding 
pans are each 5 feet in diameter and 19 inches deep. They 
make 40 to 45 revolutions per minute. The pulp then passes 
to an amalgamator or amalgamating pan 6 feet in diameter 
and 22 inches deep. This makes only 15 revolutions per 
minute. All the pans are of the Wheeler pattern. The 

* Mr. Chas. Fraser, to whom I am indebted for much information. 



166 STAMP MILLING OF GOLD ORES. 

second grinding pan is generally used as an amalgamator, and 
the pan intended particularly for amalgamation is used as a 
settler. The capacity of this plant is 15 tons per week of six 
working days. Its cost, complete, with shed, was £524. 
Worked by water-power it requires no labor in addition to 
that employed in the mill. When the latter is idle, it neces- 
sitates an expenditure of £6 per week in wages. The wear- 
ing pairts are renewed every six months and cost £20 per 
double set. 

By referring to the comparative table, given in the earlier 
portion of this paper, it will be seen that the mills have a 
general similarity in their methods. The principles of the 
milling practice of this district were derived from the experi- 
ence of Clunes. Clunes has been to the Colonies what Gilpin 
County is to the Western States ; its methods are not neces- 
sarily the best, but it has been a school wherein the bed-rock 
principles of milling have been more thoroughly learned than 
has been the case in localities where mills are more new, 
more costly, and better arranged. The mills of the Ovens dis- 
trict do not fulfill the millman's ideal. Most of them are old 
and in want of repair ; those which are new are incomplete, 
but the men in charge understand what they are doing, and 
there is reason to believe that as the mines of this Alpine 
region become developed the need of larger and better reduc- 
tion plants will be fully recognized and their working will be 
in the hands of men, many of whom will be ready to adapt 
themselves to newer methods and altered conditions. 



CHAPTER XI. 

The Usit of the Stamp Mill for Ores Unsuited for 

Such Treatment. 

The Thames gold field was proclaimed upon July 28, 1867. 
This once famous mining district, also known by the Maori 
name of Hauraki, is situated on the northeastern coast of the 
north island of New Zealand. The earliest discoveries were of 
alluvial gold, but in August of the same year the first quartz 
reef was found in the bed of Kuranui Creek, where afterward 
was opened up the Shotover claim, famous in Colonial mining 
records. Though the output has now dwindled to about 
80,000 ounces per annum, this has been in its day one of the 
richest gold fields of the world. In 1871 the output was 
330,326 ounces, valued at £1,188,708. The Caledonia mine in 
the first twelve months' operations produced ten tons of gold 
and paid £600,000 in dividends. The Moanataeri, Manukau, 
Golden Crown, and others, similarly gave remarkable yields.  

Though the mines have not yet attained any great depth, | 

600 to 700 feet being the maximum, the veins have proved to 
be far less rich than they were near the surface, and while the 
opening up of new districts has drawn away many of the best 
miners, the development of the gold field is also crippled by 
share jobbing. It is the story of many another mining camp, 
and like many other old districts the Thames is worthy of the 
further systematic development, more particularly near the 
surface, which its history invites. 

Few mining districts have had so brief but brilliant a 
record, and few perhaps have lost such a large portion of the 
gold extracted from the mines. Milling is conducted under 
the difficulties presented by ores of very variable and very 
complex composition, but so far the efforts made to overcome 

(167) 



168 STAMP MILLING OF GOLD ORE.:. 

those difficulties have been of the most elementary kind. It 
is for this reason that the tailings mills are to-day* among the 
most profitable undertakings upon the field, and that the 
mining industry of the place is at a lower ebb than its history 
has ever known. 

There are about 500 stamp heads and 360 Berdan pans at 
work in the district of Hauraki. The mills are without 
exception old and date 10 to 17 years back. The work done 
in them is most incomplete, since, notwithstanding the fact 
that the ore contains a large percentage of sulphides, there is 
no attempt made at concentration. Ordinary wet crushing is 
supplemented by the use of blankets, the washings from 
which are treated by pans, methods which do not prevent the 
tailings from carrying away a large percentage of the value 
of the ore. The tides in their ebb and flow are concentrating 
the material which they receive, and are giving a daily lesson 
to the careless millman, a lesson unappreciated by him, but 
readily understood by the proprietors of the tailings plants 
who are making money by the treatment of the sands. 

Briefly stated, the method of milling consists in catching all 
the " free gold" — that is, all the gold which can be arrested by 
means so simple as to be quite unsuited to the character of 
the ore, and allowing the remainder to go to enrich the sea 
beaches. The character of the ore and the lode formation in 
which it occurs partially help to explain a state of things 
which calls for such severe criticism. The bulk of the gold 
comes from narrow veins and extremely rich pockets travers- 
ing a decomposed andesite. The ore bodies of such a forma- 
tion must necessarily be uncertain in behavior and limited 
in extent. Like the deposits of the Nagyag and Veraspotak 
mines of Transylvania, to which they have a striking resem- 
blance,! the pockets found at the Thames are occasionally of 
extraordinary richness. Such was the lot of 2 tons 8 hun- 
dredweights, crushed by the Moanataeri in 1878, which 
yielded 14,000 ounces, or at the rate of 2^ ounces per pound. 
One bowlder of 2^ hundredweights yielded 3500 ounces. 
These crushings of small quantities of very rich ore pay the 
dividends and form a very large proportion of the value of 

* Referring to the time of the writer's examinations in 1891. 
+ As they also have to those of Cripple Creek in Colorado. 



USB OP BTAMP MILL FOR UNSUITABLB ORBS. 169 

the entire output. This is well illustrated by the returns 
obtained during six weeks in 1885 J)y the Cambria Mill. 



First fortnight, November, 18SS. .... £SG tons stone. 1B24 GSB 

1400 IbB. speclmea ore. taf» 9813 

8ei!andrortaighC,November,lS85.. . . 893 tone Htoae. 9cei TTT 

ISGSlbs. Bpeclmenore. 4631 1801 

First rortnlght, Decembor, 1885 23« tons atone. 1«00 888 

IBTOlbs. Bpeoimenore. SlIS 82Se 

The bulk of the output being thus of a value small in pro- 
portion to its weight when compared to that of a few sacks of 
" specimen ore," it is easily conceivable that it has not 
received the attention which it demanded. 

The following comparative table of five of the principal mills 
will indicate the chief features of the milling : 

COMPARATIVE TABLK. 



It would be wearisome to describe each of these mills in 
turn, so I will choose the Saxon, which is perhaps the most 
representative. 

The Saxon mill contains 32 stamps, which crush the ordinary 
ore, and one single stamp, which is kept for the treatment of 
the specimen ore. The plant bears evidence of its growth in 
the varying weight of the stamps, which is as follows : 

Ten"ltghtliead9," la2batterlesot5cach. . . .see pounds. 
Ten "heavy," In 2 " 6 " , . . . 784 " 

The shoes and dies vary in similar proportion. The rat« of 
drop averages 72 per minute, but this speed is subject to 
change. The height of the drop is from 8 to 11 inches, 
depending upon the hardness of the ore. The issue or depth 



170 STAMP MILLING OF GOLD ORES. 

« 

of discharge is about 1 inch at the time of putting new dies 
into position, and increases to a maximum of 4 inches as they 
are worn down. There is no attempt made to keep the depth 
of discharge constant. 

The single stamp weighs 7 hundredweights, or 784 pounds. 
It is given a 7-inch drop and a speed of 60 drops per minute. 
This " specimen stamp " is a curious feature of all the mills* 
The crushing capacity of the mill is from 30 to 85 loads per 
day. A load varies from 80 to 85 hundredweights. When 
not engaged in treating the specimen ore the single stamp 
aissists the other 82 in the crushing of the general output of 
the mine. 

The screen or grating used is of Kussia iron, imported from 
Swansea. The openings are round punched and 148 per 
square inch. The life of a grating will average one week or 
six working days. (No work is done in the Colonies at mines 
or mills on Sunday. ) 

The loss or consumption of mercury is at the rate of one 
bottle (of 75 pounds) per month, there being three bottles in 
use in the mill at any given time. No mercury is usod in the 
mortar boxes, save in the case of the specimen stamp ; it is 
employed, however, on the plates outside, in some of the 
''ripples'' or wells, and in the pans. Owing to the "flour- 
ing " produced by the pans the loss of mercury is excessive^ 
being 14^ pennyweights per ton of ore treated. 

Forty-two per cent, of bullion is obtained in retorting. A 
representative crushing* of 80 loads gave 479 ounces of amal- 
gam, yielding 203 ounces of bullion. 

We will now follow the ore through the different stages of 
the treatment. It is brought to the mill in carts and dis- 
charged into stalls or ore-bins behind the batteries. There is 
no rock-breaker in use, no grizzlies or sizing-bars, and the ore 
is hand fed. The feeding is badly done. The ore varies 
greatly in hardness, according to the extent to which the 
andesitic vein filling is decomposed. The quartz itself is often 
sugary and crystalline. The shoes and dies, both of local 
manufacture, are made of white hematite cast iron, that of 
the die differing from that of the shoe in being unchilled. 
The shoe is 9i inches in diameter and 10 inches high. The 

* This was in April, 189L 



USE OF STAMP MILL FOR UNSUITABLE ORES. Ill 

die is 10 inches diameter and 4 inches deep. The die is cast 
with a flange so as to keep it in position. 

The mortar boxes are faulty in design since they are too 
roomy ; there is unnecessary space in and around the dies, 
seeing that no amalgamation takes place inside. The pulp is 
discharged upon amalgamating tables 7 feet long and 4^ fret 
wide. These are in three divisions, of which the upper two 
only are lined with plates. The first length is 2^ feet, inclu- 
sive of a well 2i inches wide. This well contains mercury. 
The next length of table is 18 inches. The "ripples" or 
riffles, as they are called in the United States, are four in 
number, one only, that already mentioned, containing mer- 
cury. The other three are "blind ripples." They are 2 
inches deep. 

The gold saving is effected by amalgamated plates, by wells, 
and indirectly by blanket strakes, whose residues are treated 
in pans. There is no concentration in the ordinary sense of 
the term. 

The mortar box is not a gold-saving appliance, but merely a 
crusher. The first amalgamation takes place on the outside 
plates. These are made not of copper, but of Muntz metal. 
They are roughly cleaned up every 4 hours. The wells are of 
very little assistance ; we\l& perse are unsuited to ores con- 
taining a notable proportion of sulphides. The surface of 
the bath of mercury is continually coated with a scum of sul- 
phides, which prevents contact with the gold in the pulp 
passing over it. Here the wells are skimmed with a cloth 
every 4 hours. The mercury placed in them is squeezed once 
per week. The six wells (one to each battery) in the mill 
catch about 12 ounces of amalgam out of the weekly yield for 
the entire mill of 200 to 250 ounces. 

The "blind ripples" are not true wells, since they hold no 
mercury. They are cleaned with a scoop every half hour, the 
heavy sand and sulphides so obtained going to the pans. 

The blankets are washed every hour, this interval varying, 
however, according to the richness of the ore and the amount 
of sulphides which it contains. The washing of the blankets 
is done by one boy on each shift, three boys per day, each of 
them paid £1 per week. The blanketings go to the pans. Of 
these there are three varieties. 



172 STAMP MILLING OF GOLD ORES. 

The Berdans and one Watson & Denny treat the blanket- 
ings. The other two are working tailings. The Berdans have 
a pitch of 16 inches in 3 feet 6 inches, or 1 in 2f. The 
speed is regulated at 23 revolutions per minute. The amal- 
gam is removed every 24 hours. Instead of a ball, a drag is 
used. It consists of two parts, the "slipper" or shoe weigh- 
ing 196 pounds and the top or boss 283 pounds. They are held 
together by a key, and the surface of contact between the two 
parts is lined with cement. The shoe lasts about 4^ months. 

NUMBER AND DIMENSIONS OF PANS. 

Diameter. Depth. 

Eight Berdans 4 feet 6 inches. inches. 

Two Watson A Denny . . . 5 feet 4 inches. 2 feet 6 inches. 

One Price (local) 5 feet 8 inches. 2 feet 7 inches. 

The distribution of amalgam, which also indicates the pro- 
portion of gold saved by the various parts of the mill, is as 
follows : At the fortnightly clean-up, preceding my examina- 
tion of the mill, 200 ounces 4 pennyweights of melted bullion 
were obtained from a retort of 203 ounces, which was the 
yield from 476 ounces of amalgam, which last was thus dis- 
tributed : Plates, 223 ounces ; mercury wells, 24 ounces ; pans, 
43| ounces; 35 pounds of specimen ore, 164 ounces. 

The balance is made up by the skimmings, etc. The amal- 
gam obtained from the picked stone is obtained in the mortar 
box of the " specimen " stamp. Mercury is always added to 
the coffer or mortar box of the single stamp, and all the tail- 
ings are saved for retreatment in the Berdans. The power 
for the machinery comes from two Pelton wheels, one 6^ feet 
in diameter, which drives the stamps, and one 3^ feet in 
diameter for the pans. The water from the small wheel is 
used in the batteries. The cost of water under 60 pounds 
pressure is £3 per sluice head (60 cubic feet per minute) per 
week. It amounts to £36 per month. A small dynamo gen- 
erates the electricity which illuminates the mill at night. 
The ore treated gives an average yield of 15 pennyweights 
per long ton. 

The following figures will indicate the cost of the mill treat- 
ment at the Saxon mill per 24 hours with 33 stamps crushing 
63 tons. This includes the "specimen" stamp; 3 feeders at 
6s. 8d. per shift of 8 hours, £1 ; 3 boys, feeders also, at 5s. 



USE OP STAMP MILL FOR UNSUITABLE ORES. 173 

15s. ; 3 blanket boys at 3s. 4d., 10s. ; 3 amalgamators at 8s., 
£1 4s. ; total labor in 24 hours, £3 9s. One of the amalgama- 
tors is the superintendent or manager of the mill. The cost of 
labor is, therefore. Is. Id. per ton, while the total cost, includ- 
ing power, supplies, wear and tear, alterations to machinery, 
interest on capital, etc., is 4s. Id. per ton. At the Moanataeri 
mill, which is a larger plant, the cost is 3s. 9d. per ton. 

Coming to the examination of the comparative table it will 
be noted that the mills are for the most part small, being in 
this respect proportioned to the size of the mines. The Moan- 
ataeri mine has the most extensive workings, and owns a 
comparatively large mill. The weight of the stamps varies 
from 620 to 840 pounds. The speed varies within narrow 
limits only — from 63 to 76 drops per minute. In the height 
of the drop there is a greater disparity ; the first three mills 
are treating ore which comes from depths varying between 
200 and 600 feet from the surface, and the drop is from 8 to 9 
inches, but the last two are treating soft surface material, 
which fact explains tlie comparatively low drop of from 5 to 
6 inches. The depth of discharge or issue — the distance from 
the top of the die to the bottom of the screen or grating — in 
all the mills is regulated by the wearing down of the dies. It 
varies from nil with new dies to a maximum of 5 inches, and 
will average from 2i to 3 inches. The importance of having 
a depth of discharge suited to the particular mode of working 
aimed at and the particular ore treated is a point quite unap- 
preciated. It is curious to note that in most districts the im- 
portance of this feature of the milling is, as a general rule, 
overlooked. 

The crushing capacity varies but little for the first three 
mills, from 1^ to 3| long tons per stamp per 24 hours being the 
average. At the Moanataeri it is lower than at the Cambria 
or Saxon, by reason of a shorter drop. The Cambria crushes 
fast in proportion to the weight of the stamps ; this is due to 
the fact that it is a custom mill or "public battery." The 
slightly deeper discharge is more than made up for by the 
quicker drop. The Kuranui Hill mills — Hansen's and Comer's 
— put through a much larger quantity, since they crush soft 
surface material of low grade. 

The gratings or screens are all made of round-punched 



174 STAMP MILLING OF GOLD ORES. 

Russia iron, imported from Swansea. The fineness varies from 
148 to 180 holes per square inch. The shorter life of the 
screens in the mills treating surface ore, notwithstanding the 
lesser hardness of the material which passes through them, is 
due to the presence of the acid sulphates, derived from the 
oxidation of sulphides, which, by the reactions induced, cor- 
rode the iron of the grating. Owing to the greater speed of 
crushing, however, the gratings in Comer's mill, for instance, 
last during the passage through them of 90 tons (long) as 
against 54 tons at the Saxon. The result compares badly with 
that of other districts, and is due to the direct action of the 
acid mine waters, as well as to the more indirect reactions 
induced by those waters, when in the battery, upon the par- 
tially decomposed metallic sulphides. It may be mentioned 
here that the chemical reactions produced by the underground 
waters in the mines of this district are so marked as to suggest 
the effects of a slowly dying solfataric agency. 

The loss or consumption of mercury does not vary much, 
and is represented by one bottle (75 pounds avoirdupois) per 
month for 80 stamps. It is a high consumption, and is due to 
the flouring of the mercury by the grinding of the pans, as well 
as to the evil effects ("sickening") produced by the presence 
in the pulp of antimonial and arsenical minerals. 

The bullion is of low grade, having a fineness which ranges 
from .589 to .674 per thousand. At the Moanataeri and Saxon 
it varies as follows : Saxon, gold .6523, silver .341 ; gold .6744, 
silver .316. Moanataeri, gold .638, silver .342 ; gold .6419, silver 
.343; gold .6432, silver .349. 

Before venturing upon a further criticism of the methods 
of treatment, it will be well to give a general description of 
the character of the ore. The lodes from which it is obtained 
are small and not very regular veins, which traverse a horn- 
blende andesite which is often brecciated. The lode forma- 
tion is confined to certain belts marked by the decomposition 
of the country rock. The millstuff consists of a large propor- 
tion of the country rock, which has a hardness varying accord- 
ing to the degree of its decomposition. The ore is silver- 
bearing as well as gold-bearing. The gangue is largely quartz, 
which is arranged in veinlets and stringers through the coun- 
try rock included within the limits of the vein. Sometimes 



USE OF STAMP MILL FOR UNSUITABLE ORES. 175 

the quartz is soft and sugary. While the gold is frequently 
visible in the quartz in the form of minute threads and par- 
ticles, it is also largely associated with copper and iron pyrites, 
blende, galena, etc. It occurs coated by native arsenic. Silver 
occurs in various forms, such as pyrargyrite, proustite, argen- 
tite, etc. The two precious metals are found associated with 
tellurium, as petzite and sylvanite. Selenides are known to 
exist in the ore. Antimony in beautiful crystals of stibnite is 
often seen. 

Generally speaking, the ore has a varying hardness and 
composition; it contains a large variety of metallic sul- 
phides, and must be considered both a silver and gold ore. 
Containing from one-half to 10 per cent, of sulphides, with an 
average of from 2 to 3 per cent., it approaches the boundary 
line which divides a "free-milling" from a "refractory" ore. 

An endeavor will now be made to pass in review the chief 
characteristics of the milling. 

In none of the mills is there a rock-breaker ; therefore there 
are also no grizzlies or sizing screens. In every case the feed- 
ing is done by hand, and in most cases done badly. The 
feeding is very rough. Instead of using trained men for this 
Important work, it is left to boys who shirk the breaking of 
the big pieces of the rock, preferring to throw them into the 
feed-opening of the mortar box, where, if they stick, they are 
rammed in with a few blows from the sledge hammer. The 
results due to the absence of rock-breakers and automatic 
feeders, coupled with the bad hand-feeding which obtains in 
the mills, is seen in the excessive wear and tear of the shoes 
and dies. The average wear is at the rate of 22 ounces of iron 
per ton of ore crushed — 14.5 ounces for the shoe and 7.5 ounces 
for the die. 

The feeding, while it is irregular, is also too high. The bat- 
teries are kept almost choked up with ore, so that the stamps 
do ineffective work. 

The mortar boxes are all of the same pattern, whether em- 
ployed for the rapid crushing of soft material or the slower 
treatment of the average ore*. Seeing that no amalgamation 
is attempted inside, they are too wide. When the battery is 
simply a pulverizer the pulp should be expelled from the 
mortar as soon as it has been reduced to the size required ; 






176 STAMP MILLING OF GOLD ORBS. 

with an unnecessarily roomy mortar the pulp lingers after it 
has been reduced to a fineness which will allow of its passage 
through the grating. When amalgamation takes place inside 
this serves a purpose, but at the Thames it only permits of 
that unnecessary sliming of the ore which causes a heavy loss 
of fine particles of gold. Narrower mortars are, therefore, 
advisable. 

The gratings are all of one description. As already pointed 
out, the wear is much affected by the character of the ore. 
Material which has lain for some time in the stopes or waste 
from surface destroys the iron of the gratings by reason of 
the sulphuric acid produced by the decomposition of the sul- 
phides. The mine waters contain an unusual quantity of the 
protosulphates of iron, copper, manganese and alumina, and, 
as a consequence, when the millstuff is very wet the action 
upon the gratings is very marked. As is usual in the Colonial 
mills, the screens are arranged vertically instead of having a 
slight forward inclination. This tends to wear the lower por- 
tion much faster than the upper. The amalgamating plates 
^ U,|"^ A, ^^® made variously of copper and Muntz metal. At the Saxon 

and Kuranui mills Muntz alone is used ; at the Moanataeri 
plain copper, but at the Cambria and Comer both are used. 
In the former the top portion of the plates or tables is Muntz, 
and the lower part copper ; in the latter it is vice versa. The 
use of Muntz metal will be fully discussed later on. 

As already pointed out, the wells serve but little purpose, 
since they are not of the type (known as drop wells) that com- 
pels the pulp to pass through the body of the mercury bath. 
The small amount of gold which is collected by them only 
confirms the general experience that with ores containing an 
appreciable percentage of sulphides ordinary mercury wells 
are rendered comparatively inoperative by the formation of a 
scum over the surface of the bath. The "blind ripples" are 
still more useless, since they serve no particular purpose, 
either as arresting free gold or collecting sulphides. The 
amount of attention which is given to them is out of propor- 
tion to the quantity of sulphides and heavy sand which they 
collect. 

The blankets at most of the mills are washed at intervals 
which are too long. Tributers or lessees understand this, for 



rl^o;. 



aSE OF STAMP MILL FOR UNSUITABLE ORES. 177 

when putting through a lot of ore at any of the custom mills, 
they themselves attend to this part of the work. In such 
cases they will usually wash the blankets and skim the wells 
every half hour. The blankets cost 12 shillings per yard of 
two yards wide. They are manufactured particularly for mill 
use at Mosgeil, near Dunedin, in the south island. They last 
for three months. 

The Berdan pans have a good feature — that is, the use of a 
drag in place of a ball. It is a step in the right direction. 
The ball always remains in the lowest portion of the pan ; the 
mercury also collects there, and, as a consequence, the latter 
is ground by the former, with the formation of "floured" 
mercury, which escapes with the slimes. The drag, which is 
fixed to one side of the pan, keeps the work of grinding apart 
from that of amalgamation. 

The concentration of 'the rich sulphide minerals is unat- 
tempted save by blankets, and in rare cases by such a rude 
method as "tyes." An attempt was made to encourage this 
portion of the milling, and a Newberry-Vautin chlorination 
plant was erected at the Thames, but owing to the impossi- 
bility of obtaining a regular supply of concentrates the works 
had to be shut down. 

In summarizing the treatment which the ore undergoes at 
the mills, it is not too much to say that it is very crude and 
incomplete ; it succeeds in arresting only that portion of the 
gold which is readily amalgamated, and fails entirely in sav- 
ing the silver contents. The silver in the bullion is not the 
result of amalgamation with the silver in the millstuff, for 
the proportion of silver to gold in the bullion is that of the 
native gold of the district, which, like that of the Transyl- 
vania gold field, is of very low caratage. On the most favor- 
able estimate the treatment cannot be said to be even half 
carried out, for scarce 50 per cent, of the gold is extracted, 
leaving out of account the silver. The waste of many of the 
mills will assay about half an ounce. Much of the gold, and 
nearly all the silver, is carried out in the slimes, which, being 
deposited along the foreshore, have produced an accumulation 
of tailings estimated to amount to at least a million tons, carry- 
ing half an ounce of bullion per ton. That this is not an exag- 
gerated statement is proved by the success of the tailings mills. 



178 STAMP MILLING OP GOLD ORES. 

These are engaged in the treatment of both the old accumula- 
tions and the "waste" now being sent down. It is a sufficiently 
severe condemnation of the work done at the Thames to state 
the fact that the tailings mills, using the same methods of 
extraction as the mills themselves (that is pans, not even pre- 
ceded by finer crushing), are enabled to pay well. It is a 
sorry fact to have to record that even after this second treat- 
ment the tailings still contain a notable amount of gold and 
silver. 

The largest of these plants to treat the tailings contains 12 
Watson & Denny pans. The tailings are elevated and conveyed 
by water. This is very effectively and simply done by a small 
hydraulic elevator. The jet is five-eighths inch diameter, 
the supply or pressure pipe 2^ inches, and the elevator or dis- 
charge pipe 3 inches. The water used is under a pressure of 
only 60 pounds per square inch. The launder from the upper 
end of the elevator pipe conveys the tailings to wide strakes 
or buddies, where the poor slime is washed away and the 
rough stones picked out before feeding the material into the 
hopper of the pans. A handsome profit is being obtained. 
The total cost per ton — ^including insurance, interest on plant, 
wear and tear, etc. — amounts to 8s. 6d. per ton. 

But what is the remedy? may naturally be asked after such 
a severe condemnation of existing methods. First, to point 
out one vital error in the treatment. No reference is here 
intended to the fact that the rich silver-bearing minerals are 
allowed to go out to sea without any attempt at saving or 
concentrating them — that is no error, but pure carelessness. 
While blanket strakes, followed by pans, form a process which 
is quite ineffectual as regards the saving of the silver contents 
of the ore, it is also badly suited to the extraction of any free 
gold remaining in the pulp after its passage over the plates. 
The grinding action of the pans upon the sulphides forms 
slimes, which, sickening the mercury, causes its direct loss as 
well as spoiling its power of catching the gold by amalgama- 
tion. To make the pan treatment successful the previous 
roasting of the sulphides is necessary. 

The ore is both silver- and gold-bearing; the former is 
chiefly associated with the sulphides, the latter occurs mainly 
in the quartz, while both metals occur combined as tellurides 



USB OF STAMP MILL FOR UNSUITABLB ORES. 179 

etc. Some separation is, therefore, needed between the silver- 
bearing and the gold-bearing portions of the ore. The tellu- 
rides, etc., would be saved in the same operation as the former. 
It is suggested, therefore, that from the plates the pulp should 
pass direct, discarding the blankets, to concentrators and 
thence to the pans. The concentrators would separate out 
the silver-bearing sulphides and some of the combined gold, 
and the pulp freed from the sulphides would go to the pans, 
which would complete the extraction of any free gold 
remaining. 

This is a very obvious improvement on the present treat- 
ment. It is the so-called "combination" process. It is not 
necessary to erect an expensive plant of concentrators ; it will 
answer well enough to commence with some simple form of 
shaking table, and of this type of machine there are various 
forms to choose from, among which may be mentioned the 
shaking table of the Colorado mills, or that extensively used 
in Victoria — both simple and inexpensive. 

The process would be made more correct and the work of 
the concentrators much lightened by the intervention of 
classifiers between the amalgamating tables and the concen- 
trators. Spitzlutten or water-jet classifiers would be most 
simple, inexpensive, and complete machines for this purpose. 

The treatment above suggested would not interfere with the 
present amalgamation upon the tables ; it would be supple- 
mentary. The initial cost of such an addition would be small, 
and as the machines are practically automatic the additional 
cost of treatment would be extremely small. 

As to the handling of the concentrates, their after-treat- 
ment, that need not concern us here as it is outside the scope 
of stamp milling proper. It is to be determined by careful 
experiment whether chlorination, cyanidation, or matte smelt- 
ing is the most practicable. 

The mills of this district offer one point of particular 7^/ 
interest, namely, the use of Muntz metal on the amalgamat- v_-J '^A )}/ 
ing tables. The composition of Muntz metal is 60 per cent. T(^ L-"'' 
copper and 40 per cent. zinc. Its first introduction into mill ' ' ^ 

use took place in 1875. At that time copper plates were not 
obtainable in sufficient quantity, and the Thames being a sea- 
port the local ironmongers, who imported Muntz for the pur- 



180 STAMP MILLING OP GOLD ORES. 

pose of sheathing vessels' bottoms, sold it in place of the 
copper. The first trials were very satisfactory, its use spread, 
and it has now largely replaced the simpler metal. Sheets of 
proper dimensions for mill use have not been introduced; 
those employed at present a^e generally of insufficient size, 
and require patching together so as to fit the width of the 
tables. The sheets used are also too thin for the purpose, 
being of the thicl^ness known as No. 18. They last, however, 
from three to five years. 



Mills in the Thames Dlstrlot. 

Silver-plated copper is not used in the mills of this district, 
hence the comparisons to be made will be confined in this 
instance to Muntz metal and plain topper. In many respects 
the facts noted will apply also to the better class of amalga- 
mating plates. 

Muntz metal, as used for amalgamation, has the following 
characteristics. It does not absorb amalgam like copper. The 
latter has to be well coated with gold amalgam before it. 
does any good work, and in a new mill it is always advisable 
to allow the copper to become thoroughly amalgamated before 
the plates are cleaned. Muntz metal lias very little absorbing 
power over the mercury — that is, the amalgamation is, as 



USE OF STAMP MILL FOR UNSUITABLE ORES. 181 

compared to that of copper, very superficial. In practice 
several results follow. First, the clean-up is much facilitated, 
for the amalgam formed on the Muntz is very readily de- 
tached, not requiring the more laborious cleaning up demanded 
by copper plates. A steel tool is never used, rubber being 
always suflScient. Secondly, test crushings are more reliable. 
Gold will not collect upon a poor copper plate as it will upon 
a rich one, so that in practice the yield from a lot of ore will 
be much affected by that of the previous lot. It is not pos- 
sible to scrape a copper plate closely without injuring its 
amalgamating capabilities. It is not so with Muntz metal, 
which can be readily deprived of its previous gain of gold 
amalgam without impairing its efficiency. It is, therefore, 
particularly suited to custom mills. 

On the other hand, for very rich ore Muntz is not advisable, 
for " there is no body in it," as the Thames millmen say — 
that is, it is sooner saturated with gold amalgam than is the 
copper. In the same way silver-plated copper will carry more 
amalgam than the plain copper. This is a disadvantage to be 
partially overcome by frequent cleaning up, a remedy, how- 
ever, which does not get over the defect. 

When ores contain minerals injurious to the mercury Muntz 
metal is preferable. It is noted that in the presence of base 
minerals "sickening" of the mercury does not occur as with 
copper plates. The explanation is to be found in the zinc, of 
which it is partly composed, which, by reason of a feeble 
galvanic action which is set up between it and the copper, 
electrolizes the water and liberates the hydrogen, which, 
being in a nascent state, exerts a powerful reducing effect, 
somewhat after the way of the so-called " hydrogen-amalgam" 
process. In practice there results the fact that Muntz metal 
plates are easier to keep in good order than the copper. When 
the millstuff contains a notable percentage of the heavy 
sulphides, particularly such as are directly injurious to amal- 
gamation and "sicken" the mercury, it is seen that the 
Muntz is hardly affected. The "verdigris" of the millmen 
is not formed upon Muntz metal plates. At the Saxon mill it 
required over a jar (7 pounds, costing 23 shillings) of potassium 
cyanide to dress the copper plates, and so keep them in order ; 
it was found upon introducing Muntz metal that half a bottle 



182 STAMP MILLING OF GOLD ORES. 

(one bottle costing 6s. 8d.) of sulphuric acid would suffice 

during the same length of time, viz, one month. 

^1 On the other hand, it is known that for highly acid ores, 

'^'^^'^ A such as certain heaps of waste ("mullock tips" or dumps), 

^ \ • ' 7 which contain much sulphuric acid, resulting from the oxida- 

/ Op' "' ' tion of pyrites, the copper is to be preferred. Under such 

^^ conditions a heavy scum is formed upon the Muntz, while the 

acid tends to keep the copper clean 

At the Cambria, which is a custom mill, both varieties of 
plates are used, Muntz for the top and copper for the lower 
portion of the tables. This is done to meet the requirements 
of different classes of ore. At the Comer mill the arrange- 
ment is reversed, the top plate being of copper. 

The result of the experience of the Thames mills has been, 
therefore, to recommend the use of Muntz metal for amalga- 
mating plates where poor ore is being crushed, also in custom 
milling and where the ore is charged with minerals which 
injuriously affect the mercury. For material containing acid 
waters or for very rich ore, unaccompanied by a large per- 
centage of sulphurets, copper is to be preferred. 

In general it may be added further that Muntz metal is the 
cheaper of the two ; it lasts longer and requires less attention. 

It would be well to try Muntz metal in other districts where 
somewhat similar conditions obtain. It possesses two certain 
advantages : it wears better than the ordinary plates and it 
facilitates a rapid and complete clean-up. This should recom- 
mend it to custom mills. Its other advantages must depend 
upon the character of the particular ore whose gold contents 
it is called upon to amalgamate. 

In dressing new Muntz metal plates the following are the 
steps to be taken : Rub the surface of the new plate with fine 
clean sand to get it mechanically clean, then wash it with a 
weak (1 to 6) solution of sulphuric acid to make it chemically 
clean. Then start to rub in a little mercury, rub in one place 
"until it bites" — that is, the plate becomes amalgamated. 
Give a circular movement to the flannel or mop. Once 
started the amalgamation spreads in ever-widening circles. 

Every mining district has a lesson to give, every mill some 
suggestion to offer ; in the case of the Thames the point of most 
interest is undoubtedly that which we have last considered* 



USE OF STAMP MILL FOR UNSUITABLE ORBS. 183 

Since the above was written I have received an account of 
a trial crushing made at one of the mills of the Thames. The 
milling practice of this district has been described, the 
methods employed have been very severely criticised, and 
the mills condemned as totally unsuited to the reduction of 
the ores whose values they were intended to extract. In 
estimating the percentage of the saving, I stated that "the 
treatment cannot be said to be even half carried out, for 
scarce 60 per cent, of the gold is extracted, leaving out of 
account the silver." From the columns of the local newspaper 
I learn that the Director of the Government School of Mines 
has been investigating the question, and that recently a test 
mill run was carried out at a representative mill (the 
Cambria), which is under the charge of Mr. T. A. Dunlap, 
certainly one of the most experienced of the local mine 
managers. 

The results indicate that my criticisms were fully warranted. 
Twenty-seven tons of ore were crushed. Each ton contained 7 
ozs. 18 dwts. 19 grs. of gold, and 9 ozs. 14 dwts. 2 grs. of silver, 
worth, together, £333 4s. 6d. The extraction was as follows : 
From amalgamating plates, £30 Os. 7d., or 3.3 per cent. ; from 
pans (Berdans), £432 16s. 5d., or 48.2 per cent.; total, £462 
16s. Od., or 61.6 per cent. 

It will be remembered that such concentration as is effected 
in the mills of this district is accomplished by the use of 
blankets, the washings from which are subsequently treated 
in pans. The account goes on to say: "Considering the 
highly mineralized and refractory character of the Alburnia 
ore, the percentages of saving effected at the Cambria battery 
are remarkably high, and must be the result of much skillful 
manipulation ; nevertheless, it seems a pity to see so large a 
loss of bullion after the time and expenditure incurred in 
finding, mining, and carting the ore." 

The "skillful manipulation" is sadly misdirected, and the 
"loss of bullion" is a woeful result, due to a foUow-my- 
neighbor policy. The great axiom of successful milling is to 
adapt the reduction process to the character of the ore treated. 
This is the reason of the " variations" in the milling of gold 
ores. The Thames affords an instance of the disregard of this 
elementary law. 



CHAPTER XII. 

The Stamp Mills of Otago, New Zealaio). 

The province of Otago, in the south island, was the first 
gold-mining district opened up in New Zealand. Its history 
dates from the discovery in June, 1861, of the golden gravel 
which made Gabriel's Gully a household word in the Colonies. 
It has ever since continued to bQ more of a rich and extensive 
alluvial field than a quartz-mining country, but the last few 
years have seen considerable progress in the development of 
the quartz lodes whose importance is becoming better recog- 
nized. Up to March, 1892, Otago, which has an area of 20,- 
000 square miles, has produced 4,889,563 ounces of gold, 
having a value of £19,310,707, or $96,553,535. For the year 
ended March 31, 1892, the yield amounted to 105,531 ounces, 
worth £423,527, or $2,117,635. 

The milling practice has largely followed that of the older 
mining centers of Victoria, but altered conditions have in- 
duced some variations which have given it certain charac- 
teristics which merit our attention. The three mills whose 
figures are given in the comparative table which follows are 
fairly typical. The Phoenix battery at Skippers is one of the 
best known in New Zealand, and nestles at the foot of the 
snowy ranges of the Southern Alps. The Premier at Mace- 
town is an old mill shortly to be replaced by a larger plant. 
The last on the list is a mill at Nenthorn, a mining camp 
which, though of comparatively recent origin, has already 
passed through many vicissitudes of alternating failure and 
prosperity. 

The Phoenix mill, at the head of the Shotover River, was 
one of the pioneers in the utilization of electricity for the 
transmission of power. The plant was erected in 1884, and 

(184) 



THE STAMP MILLS OF OTAOO, NEW ZEALAND. 185 

consists of two Pelton wheels under a head of 180 feet, which 
drive two Brush dynamos connetted by two No. 8 B. W. G. 
copper wires, nearly three miles Jong, to a Victoria motor, 
which in turn supplies the power to run 30 stamps, together 
with a rock-breaker, and to light the buildings. 

COMPARATIVE TABLE. 



Each stamp complete weighs 800 pounds. The speed varies 
from 75 to 80 drops per minute. The height of the drop is 
from 7 to 8 inches. The depth of discharge is variable. At 
the commencement of the month and immediately after the 
clean-up, it is 2^ inches. The maximum distance from the 
bottom of the grating or screen to the die or false bottom is 
reached when the dies are worn down and it is then 4i inches. 
The average is therefore 3^ inches. The mill crushes 460 tons 
per fortnight of twelve working days, or at the rate of a little 
under IJ tons per stamp per 24 hours. Two kinds of gratings 
or screens are in use. Wire cloth is chiefly employed, but 
when the supply runs short the ordinary round-punched 
Kussia iron is substituted. The holes in the two cases are of 
similar size, but the number of them per square inch is 324 in 
the one case and only 140 in the other. The pyrite which the 
ore contains has been proved to be of very low grade and no 
after-concentration is tlierefore attempted. The bullion is 
930 fine. The last two shipments, previous to my visit, were : 
.922^ fine, worth £3 18s. 3d., and .933 fine, worth £3 18s. 3d. 
The loss or consumption of mercury was 90 pounds avoirdu- 
pois during the crushing of 3107 tons of ore, being at the rate 
of 8.4 pennyweights troy, 

• Round-pun cheil Russia iron. 



186 STAMP MILLING OF OOLD ORBS. 

The gold saving is done by the mortar box and by blanket 
tables, the residues from the one and the washings from the 
other both undergoing supplementary treatment in an amal- 
gamating barrel. The method of milling is particularly 

r% simple. The ore passes through a rock-breaker and is fed by 

>>> an automatic arrangement into the battery. The feeder, 

'^"^ . ^^o t'hough a very elementary contrivance, does very good work 
^^-t^ and is worthy of description. When there is insufScient ore 
on the die the tappet strikes the head of an iron rod whose 
lower end is fastened to a chute leading to the ore-bin. The 
bottom of the chute passes upward under the ore and the 
shock communicated by the stamp through the tappet to the 
ore causes it to move downward into the feed-hole of the 
battery. This is the idea which underlies most of the more 
complicated machines which are used for the same purpose. 

The mortar box has a depth from the lip or level of dis- 
charge of 9i inches, distributed as follows: Distance from 
bottom of grating to top of die, 2^ inches ; thickness of die, 
4 inches ; false bottom 3 inches. This false bottom consists 
of two sets of four bars, which are placed under the dies and 
are packed in between with sand. Each of these bars is 3 
inches square and has a length of 2 feet and 5 inches. The 
space between each bar and the distance of the outer ones 
from the side of the coffer or mortar box is in each case 3^ 
inches, the interior width of the coffer being 15^ inches. The 
whole arrangement is simply an expedient for surmounting 
the difficulties presented by a mortar whose shape is unsuited 
to the nature of the milling required by the ore. Before 
starting the mill the coarse sand from the previous clean-up 
is packed between and around the dies. The order of the 
drop of the stamps is 3, 6, 1, 4, 2. No mercury is added to 
the ore when in the mortar box, the gold being arrested by 
the action of gravity alone. 

On leaving. the mortar box the pulp has three drops, mak- 
ing 18 inches in all, before it reaches the uppermost blanket. 
This fall serves the purpose only of spreading the material. 
T^ere are no amalgamating tables, and the pulp passes imme- 
diately over the blanket strakes. These last have a length of 
18 feet and a width of 6 feet, subdivided into four partitions. 
The gradient is seven-eighths inch per foot. The quantity of 



THE STAMP MILLS OP OTAQO, NEW ZEALAND. 187 

water used amounts to about four American gallons per stamp 
per minute, and is supplied to each 15 stamps by two 1^-inch 
pipes under a head of 25 feet. The blankets are freed from 
the gold and heavy sand which they collect by being washed 
in a tub of water. A vigorous movement through the water 
is given to them. The top row (there are four rows) of 
blankets is washed every hour and the lower ones at longer 
intervals, depending very much upon the richness of the ore. 

The blanketings, or residues from the washing, are removed 
from the tub when a certain amount has been accumulated, 
and are conveyed in buckets to a barrel, 6 feet by 4 feet, 
having a capacity of 120 gallons, equal to about one ton of 
blanket residues. In running 25 to 30 stamps with the 
average grade of ore — 15 pennyweights — to the ton, the 
supply for a barrel is obtained each day. Warm water is not 
used, nor are pieces of iron introduced, as is often done in 
California, with the idea of assisting the grinding, but with 
the result of flouring the mercury. Experiments are being 
made with the use of bluestone. A full bottle* of mercury is 
added. The barrel turns at a speed of 20 revolutions per 
minute. 

When the amalgamation is considered to be completed, 
usually after 24 hours, the material is emptied into a vat 
underneath from which it is slowly fed by a running stream 
of water to a shaking table of the Rittinger type, and having 
dimensions of 8 feet length and 2 feet width. Below the table 
there* are a few pieces of copper plate, which, however, serve 
but little purpose. The collection of the amalgam from the 
contents of the barrel by the shaking table occupies from 2^ 
to 4 hours, depending upon the rate of feeding which again 
varies directly with the heaviness of the pulp. The material 
— amalgam, pyrites, heavy sand, etc. — thus roughly concen- 
trated is placed in enameled iron buckets to be further washed 
by hand in a "dish" or pan. This latter utensil has the shape 
of the ordinary friend of the prospector, but it is of copper, 
and, its surface having become amalgamated by frequent use, 
it readily collects the amalgam. 

On examining the tailings below the shaking table, the 

* In Australia a bottle of mercury contains 75 pounds, and in America a flask 
holds 7Q% pounds avoirdupois. 



188 STAMP MILLING OF GOLD ORES. 

writer found that they contained a large amount of floured 
quicksilver. It will be noted by reference to the comparative 
table that a loss of 8.4 pennyweights of mercury per ton of 
ore crushed is there shown. This is more than the average, 
and is due undoubtedly to the excessive speed of the amalga- 
mating barrel, whose 20 revolutions per minute should be 
diminished to about 14. At Clunes, in Victoria, the speed is 
16 revolutions with a much smaller barrel. At the monthly 
clean-up the battery residues are roughly screened on a riddle, 
and the larger bits of quartz removed previous to adding the 
remainder to the blanket washings, to share with them the 
treatment in the barrel. 

Before entering into a discussion as to the effectiveness of 
the milling, the following additional details will be of value : 
Two kinds of screens or gratings are used — wire cloth and 
round-punched Russia iron. The friable quartzose character 
of the millstuff makes the former preferable. It is easier 
placed in position upon the screen frame, it has a somewhat 
longer time of wear, and a much greater area of discharge. 
The punched iron gives finer crushing at first, before the burr 
is worn off, but afterward becomes easily choked up. The 
short life of the gratings — a week for the wire, and slightly 
less for the punched iron — is not due to anything in the ore 
itself, which is a comparatively clean quartz, but to the frag- 
ments of wood ( from mine timbers) which to a more than usual 
extent find their way into the millstuff. They choke up the 
gratings, which by reason of the pressure of the water and 
pulp thus held back are caused to burst. This is notably the 
case with the punched iron, which discloses lines of weakness 
along the vertical divisions made by the press employed in 
their manufacture. 

The wire cloth, No. 18 mesh, costs 9 shillings per yard. It 
is sold in pieces 30 yards long by 2 yards wide. The grating 
cut out of this is 2 feet 6 inches by 10 inches. On the other 
hand, 120 punched Russia iron gratings cost £17, or 2s. lOd. 
apiece, as against 2s. 2d. for the wire cloth. The expenditure 
under this head amounts to £5 per month for the 30 stamps. 

In the place of blankets green baize is used; it costs 3 
shillings per yard and has a time of service varying from three 
to four months. The expenditure per month is equal to £7 



THE STAMP MILLS OF OTAGO, NEW ZEALAND. 189 

for the entire mill. Those which are least worn are always 
placed in the first row. The washing of the blankets is done 
by boys, the wages being 7i shillings per shift. One stout lad 
will do the work demanded by three batteries, but cannot 
manage the washing of the blankets belonging to 4 batteries 
or 20 heads. 

The treatment of the ore as carried out at this mill is inter- 
esting on account of its simplicity. Before questioning 
whether it be correct in principle we must examine the ore. 
This is essentially of the free-milling type and is broken from 
a large quartz lode traversing schists. The quartz often has 
a laminated or ribbon structure, which renders it readily 
broken. Inclusions of country rock are common. Pyrites or 
other sulphide minerals are present in very small proportion, 
from about one-half to three-fourths per cent. only. Concen- 
trating tests have shown that the best product will contain 
only 10 pennyweights of gold, giving a value too low for treat- 
ment in this particular locality, and on the small scale 
required by the necessities of the case. The gold occurs free. 
Ore containing more than the usual percentage of pyrites is 
generally below the average grade. The gold is not therefore 
notably associated with the pyrites. It is usually coarse and 
often visible. Quartz which in the stopes does not on exam- 
ination show occasional specks of gold is generally of low 
tenor. On being crushed the matrix of quartz readily separ- 
ates from the particles of gold. In the mill it would be 
expected that the coarsest gold would be found to remain in 
the mortar box and that that obtained from the first row of 
blankets would be less fine than that washed from the bottom 
row. Such is the case ; that found in the batteries on clean- 
ing up is very coarse indeed, pieces weighing 5 to 8 penny- 
weights being occasionally obtained. 

The mill is illuminated at night by electricity, and on ex- 
amining the blanket strakes by the aid of a powerful arc light 
the yellow particles of gold can be distinctly seen scattered 
over the green baize. The clean-up indicates the distribution 
of the gold saving ; 230 tons yielded 691 ounces of amalgam. 
Of these 691 ounces, 270 came from the blanket washings 
and 421 from the residues in the mortar box. Thus about 61 
per cent, remained in the battery. On retorting, 313 ounces 



190 STAMP MILLING OF GOLD ORES. 

of bullion resulted ; and upon melting, this was reduced to 
301 oz. 4 dwts., worth £3 19s. 3d per ounce. 

The two most striking features of the method of treatment 
are, first, that no mercury is used in the mortar-box, or 
indeed, in the mill proper, its use being confined to the after- 
treatment ; and, secondly, that the gold saving is effected by 
gravity alone. This system is borrowed for the most part 
from the mills of Olunes, Victoria, and was by them in turn 
derived from those of Nagyag and Verospotak, in Hungary. 
It will be allowed that the more simple a mill treatment is, 
the better, because it is also usually cheaper. Another mill- 
ing axiom is that the treatment should vary according to the 
nature of the ore. Here, if the methods employed are 
elementary, the character of the millstuff is no less strikingly 
simple. Whether the mill succeeds in the extraction of a 
proper percentage of the value in the ore is then the question. 
In this case repeated assays of the tailings from the Phoenix 
mill prove that excellent work is being done. The composi- 
tion and character of the ore justify the entire replacement 
of the ordinary copper plates by blankets, and the successful 
extraction confirms this. 

In milling, as in mining, we are apt to generalize somewhat 
hastily, and the good work done by his mill has made the 
manager of the Phoenix, to whom I am indebted for courtesies 
received, an enthusiastic advocate of blankets and an equally 
pronounced enemy of amalgamating plates. He communi- 
cated to me the following results of an experiment carried out 
at his mill : Two five-stamp batteries were supplied with 80 
tons each of the same kind of ore ; No. 1 battery was pro- 
vided with mercury inside the coffer or mortar box, with 
copper amalgamating tables outside, with mercury wells, and 
finally two rows of blankets. No. 2 battery was supplied 
with no mercury and was supplemented by blankets alone. 
The results of the test showed that 8 ounces (or 2 penny- 
weights per ton) more were obtained by No. 2 than by No. 1. 

In condemning copper plates the manager equally objects 
to the use of mercury in the rest of the mill, and would con- 
fine its employment to the final collection of the gold in the 
blanket washings. As a case in point and to confirm the cor- 
rectness of his ideas, he instanced the Invincible mill, on the 



THE STAMP MILLS OF OTAGO, NEW ZEALAND. 191 

other side of the same range of mountains, where the gold 
saving was done by the mercury in the battery itself, by 
wells, by amalgamating tables, and, lastly, by blankets. On 
ceasing to add mercury to the ore in the mortar box it was 
found that more gold was saved. 

The two instances, at the Phoenix mill and at the Invin- 
cible, merit careful examination. Take the second case first. 
It so happened that I had visited the Invincible mill, though 
it was then idle. It seems no wonder to me that the addition 
of mercury to the ore when in the mortar box did not improve 
the gold saving — that it indeed caused a loss ; for the mortar 
boxes are merely square iron boxes in no way modified to do 
the particular work required of them. The explanation of 
the results above quoted is to be found in the fact that the 
mortars were not designed of a shape adapting them for amal- 
gamation inside, and there was no opportunity given to the 
amalgam to collect out of reach of the falling stamps, but, on 
the contrary, the quicksilver added was subjected to a violent 
agitation which caused it to be floured — that is, broken up 
into a myriad of small globules. These last are readily borne 
away by the water, and, escaping with the tailings, also take 
with them a certain amount of gold with which they may 
have come in contact. 

At the Phoenix mill the experiment quoted is vitiated in a 
similar way. You cannot make a mortar box a successful 
amalgamating machine by the mere addition of quicksilver. 
The batteries of this mill are rectangular in section, with ver- 
tical ends and sides, and are in no way adapted for inside 
amalgamation. To make a fair comparison between the 
effectiveness of amalgamation as against blanket saving, it is 
necessary to have the two types of batteries, one roomy and of 
particular shape, the other narrow and severely rectangular, 
whose construction has kept in view their suitability to the 
two modes of milling. 

But there is no suggestion intended to be made that 
blankets could be advantageously replaced at the Phoenix 
mill by amalgamating plates. Different ores require different 
modes of treatment. Generalizations are always dangerous. 
Now, if blankets will arrest your gold, it is obviously not 
advisable to use an expensive chemical like quicksilver or to 



192 STAMP MILLING OF GOLD ORES. 

employ an apparatus so troublesome as copper amalgamating 
plates. Here at the Phoenix the mode of milling is of unusual 
simplicity, but it is suited to the ore whose gold contents it is 
intended to extract; and in saying that, one has made the 
best commendation of any particular system of treatment. 

Passing on to another mill, the Premier, at Macetown, is a 
much smaller plant, but is engaged in the treatment of a 
somewhat similar ore by slightly modified methods. The mill 
consists of 10 heads, weighing 760 pounds each. The speed 
varies from 75 to 80 drops per minute. The height of the 
drop has a maximum of 9 and a minimum of 6 inches, accord- 
ing to the hardness of the millstuff. The issue or depth of 
discharge averages 6^ inches, from 6 inches when the dies are 
new to 7 inches when they become worn down. The depth is 
regulated by the insertion of a '' blind " or blank piece of sheet 
iron inside the screen frame, which increases the issue at the 
start when fresh dies have been placed in position. As the 
dies wear down, a smaller similar piece is inserted and finally 
the full depth of the screen is utilized. The capacity of the 
mill is 65 to 70 tons per week of six working days. The grat- 
ing or screen is of round-punched Russia iron, having 180 
holes per square inch. The bullion is .949 fine, and the amal- 
gam yields on retorting 30 to 38 per cent. 

The gold saving is done by the mortar box, to which mer- 
cury is added, by the copper plates on the tables outside, by 
wells, and finally by blankets, supplemented by a Berdan pan. 
There is no rock-breaker in use ; the feeding of the batteries 
is done by haiid. The mortars of the two 5-head batteries are 
of diflFerent patterns. One is more roomy than the other, and 
therefore discharges the pulp more slowly. Seeing that 
amalgamation inside the mortar box is desired, the millman 
is right in preferring the wider coffer, since it gives more 
shelter to the particles of gold and mercury, and thereby 
better favors the amalgamation. On examination I found, as 
was to be expected, that the pulp issuing from the wider 
mortar was finer than that of the other, though the same 
kind of screen was used in both. 

Contrary to the usual practice the blankets precede the 
copper plates. On being discharged from the battery, the 
pulp has a drop of 22 inches before it falls upon the first row 






THE STAMP MnXS OF OTAGO, NEW ZEALAND. 193 

of blankets. This drop serves no purpose except that of 
spreading the 'material over the surface of the blanket tables. 
These last are 12 feet long and 4 feet 3 inches wide, divided 
into three longitudinal partitions. They slope 1^ inches per 
foot. The blankets succeed each other in three equal lengths. 
The first or top row is washed every hour, the second every 
alternate, and the third every third hour. Then follow the 
copper-plate amalgamating tables, 9 feet long by 4 feet wide. 
The total length is subdivided by five wells, one each at the top 
and bottom, and three others at equal distances between. Of 
the five, three only are supplied with mercury. They are 3 
inches wide and only one-half inch deep. 

The residues from the blankets are shoveled from one tub 
into a second, from which they are fed by a running stream 
of water into a Berdan pan of 4 feet diameter. Instead of 
the ordinary ball, a suspended muUer, called the "drag," 
placed at one side of the pan, does the grinding. This modi- 
fication keeps the grinding and amalgamation separate, 
thereby preventing unnecessary flouring of the mercury. 

A copper plate, 4 feet 8 inches by 2 feet, is placed below 
the Berdan with the view of arresting any amalgam escaping 
in the slimes. At the lower end of the plate there is also a 
mercury well. The Berdan makes one revolution for every 
three drops of a stamp — that is, 25 revolutions per minute — 
when the average speed of 75 drops per minute is being 
maintained. 

Of the total amount of amalgam obtained, 60 per cent, is 
found inside the mortar and 33 per cent, in the blanket wash- 
ings. The copper tables save the remaining 7 per cent. It 
was found that by using copper plates below the blankets as 
against a fourth row of blankets, about 5 per cent, more 
amalgam was obtained. This last observation is of interest 
as proving what might otherwise be inferred, namely, that 
blankets are particularly suited to the saving of coarse gold 
just as plates are particularly adapted to the arresting of fine 
gold. The third mill on the list is a comparatively new 
machine and is of American design. It is to be regretted 
that the irregular yield of the Nenthorn mines has prevented 
any reliable tests being made with the view of comparing it 
with the older plants. 



194 STAMP MILLING OF GOLD ORES. 

It will be noticed that at none of these mills is there any 
attempt to concentrate the sulphide minerals. As a rule, the 
Phoenix being a notable exception, the pyrites of the Otago 
lodes yield a very good grade of concentrates. There is, how- 
ever, no chlorination or smelting plant in the province, and 
any concentrates obtained have to be shipped to Australia for 
treatment at a <JOst and delay proportionate to the distance. 
That fact goes far to explain the neglect of this part of the 
milling. 

Before concluding it will be well to glance briefly once more 
at the two older mills. Both the Phoenix and Premier lodes 
carry ore the gold of which is coarse and free. This explains 
the comparatively crude and very simple method of treatment. 
Under such favorable conditions blankets are very effectual 
contrivances for arresting the gold. This system of gold 
saving is of very early origin. It was used in America before 
the discovery of gold in California. The mining districts of 
the Sierra Nevada borrowed it from the miners of Georgia, 
and they in turn owed it to those of Verospotak and Nagyag, 
in Hungary. It came back eastward when the discovery of 
the Gregory diggings started the mining industry of Colorado. 
It was derived by the millman of Otago, from the mills of 
Clunes, in Victoria, which, like those of the United States, 
borrowed it from Europe. 

Blankets mark the infancy of milling and belong to the 
gossan stage of mining. They can only survive those changes 
in the ore which accompany the increased depth of the mines 
when that ore remains, as rarely happens, unaccompanied by 
much pyrites and that pyrites not too closely associated with 
the gold. It will be noted that the Premier mill uses less 
water than the Phoenix, due to the fact that the blankets of 
the latter have a less gradient and a larger surface. At the 
Premier mercury is added to the ore in the battery, while at 
the Phoenix this is not done. The latter is probably the 
more correct practice. The gold is coarse and free, and, other 
things being equal, when a large percentage can be arrested 
by the blankets it is probable that the still coarser particles 
which remain inside the battery would be caught there by 
reason of their own gravity and without the aid of mercury. 



THE 8TAMP MILLS OF OTAGO, NEW ZEALAND. 195 

In both mills the final extraction of the gold from the blanket 
washings is roundabout and clumsy. It should be possible to 
treat the residues without so much manipulation. 

I conclusion, while it may appear that the mills of Otago 
have but little that can be advantageously imitated by those of 
Colorado or California, for the sufficient reason that they are 
adapted to the treatment of an ore of a very simple character, 
yet the examination of their modes of working can be of 
value to the American millman in causing him to ponder over 
the why and wherefore of many parts of his own practice 
whose advantage he is too ready to accept without previous 
questioning or consideration. 



CHAPTER XIII. 
A Review of Australian Practicb, 

Wet methods of ore reduction still hold the field in the 
great gold regions of the southern hemisphere. In America 
it is already otherwise. As the railway octopus slowly spreads 
out its iron tentacles there is a tendency for the smelter to 
supplant the mill, for fire reduction processes to replace wet 
methods of gold extraction. In Australia, however, the 
smelter as yet rules only over the limited territories of the 
silver regions. Elsewhere the stamp mill reigns supreme. 
This condition of the metallurgical practice of the colonies is, 
of course, in the first place due to the character of the gold 
ores themselves, for if the treatment appear primitive it is 
due to the fact that in most of the large producing mining 
centers the very simple nature of the millstuff has not called 
for complicated methods, but it is also traceable to the absence 
in convenient neighborhood to the gold mines of such deposits 
of copper or lead as would render profitable the smelting of 
siliceous gold ores. Simple gold-bearing quartz, such as is 
ordinarily termed " free milling," yields the bulk of the gold 
production of Australasia. 

Victoria, which produces 40 per cent, of the total yield, is 
as yet mining and milling only the simplest ores. There are, 
it is true, valuable deposits of refractory material, particularly 
in the northeastern part of the colony, but at present they 
make no noteworthy contribution to the general output. 

Somewhat similar conditions obtain in New South Wales, 
where gold was first discovered in Australia, and which now 
yields 7 per cent, of the total production of Australasia. In 
the central parts of the colony, in the Lachlan and Bathurst 
districts, gold is being obtained from millstuff carrying a 



A REVIEW OP AUSTRALIAN PRACTICE. 197 

notable percentage of pyrites and other sulphides, requiring 
some grinding process after ordinary plate amalgamation. 
Around Armidale, in the northeast, the association of anti 
monial minerals with the gold has often proved a source of 
annoyance. Notwithstanding these and other exceptional 
occurrences the yield of New South Wales as a whole can be 
said to come from comparatively docile ores and by means of 
simple methods of extraction. 

In Queensland, which produces nearly as much gold as 
Victoria, many metallurgical problems have arisen within 
recent years. In some cases a successful solution has been 
reached. At Mt. Morgan, for instance, barrel chlorination 
has proved triumphant over difficulties which stamp milling 
was powerless to overcome. At Charters Towers, at the 
present time the leading gold producing district of Australia, 
the ore treatment has presented many difficulties. Ordinary 
stamp milling, followed by a whole paraphernalia of concen- 
trators and pans, holds the field, but its position is being 
strongly assailed by chlorination. At Ravenswood, a neigh- 
boring locality famous for its refractory ores, cyanidation is 
struggling against many obstacles. 

New Zealand, producing nearly 15 per cent, of the Austral- 
asian gold output, has a great variety of ores, a fact which 
would suggest a more marked diversity in the methods of 
treatment than actually obtains. On the west coast of the 
south island, at Reefton more especially, the gold comes from 
quartz ores of comparative simplicity, and the stamp mill 
does fairly good work. The same may be said of the southern 
province of Otago, though in the latter region much value has 
been forever lost by an omission to save the pyrites by con- 
centration, after ordinary battery and plate amalgamation. 
In the north island, at Hauraki or the Thames, the ores are 
very complex and the stamp mill is in vain attempting to do 
satisfactory work under, almost hopeless conditions. Wet 
methods of reduction have survived long after they were fitted 
to obtain an economic extraction of the values in the ore. In 
the neighboring districts the cyanide process has been em- 
ployed with very varying success. 

By way of summary it may therefore be said that the mill- 
stuff of Australia is for the most part a simple quartz, con- 



198 



STAMP MILLING OF GOLD ORES. 



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A REVIEW OF AUSTRALIAN PRACTICE. 199 

taining from 0.5 to 2 per cent, of pyrites, mixed with a vary- 
ing proportion of country rock — slate, schist, or sandstone — 
but that within its wide confines there occur ores in every 
gradation from the most docile to the most refractory. 

The accompanying table will give at a glance the general 
characteristics of the stamp mills of six of the most represen- 
tative mining centers. It will serve as a text for the remarks 
which follow. 

The first stamp mill erected in Australasia was that of the 
Port Phillip & Colonial Gold Mining Company, which com- 
menced work at Clunes in May, 1857. The stamps, 20 in 
number, had square heads. The process of treatment at the 
commencement of operations was largely founded upon that 
of tin dressing as conducted in Cornwall, but the enterprise 
and good sense of the management was abundantly shown 
in the succeeding years by the changes made in their methods 
and by the modification of the treatment to suit the ores of 
their own mines. 

The ore broken from the veins at Clunes is of a very simple 
kind. It is a white, rather friable, quartz traversed by 
numerous fractures and cavities in which the gold occurs and 
from w^hich it is readily detached by the blow of the stamp. 
Metallic sulphides are present in very small percentage and 
do not appear to be intimately associated with the gold. 

The methods employed in this district are of the simplest 
kind. Blankets and mercury traps (or wells) do the work 
usually accomplished by amalgamated copper plates. The 
concentration of the pyrite is done by Cornish buddies of an 
improved type. The concentrates are roasted in an ordinary 
reverberatory and then treated in a Chilian mill, at a total 
cost of $9 per ton. The work accomplished by the mills has 
been of the most satisfactory character. The Port Phillip 
used to extract, as shown by accurate sampling and careful 
tests, from 85 to 90 per cent, of the value in the ore. There 
are few milling centers where there is so little room for 
adverse criticism. 

At Bendigo the ore is also a comparatively simple quartz, 
but it is harder than that mined at Clunes, and it contains an 
increased percentage of pyrites and other sulphides. The 
finer crushing which the ore requires indicates these condi. 



200 STAMP MILLING OF GOLD ORBS. 

tions. The character of a millstuif is, however, also deter- 
mined by the amount and kind of country rock mixed up in 
it. The Clunes ore is fairly clean quartz, while that of Ben- 
digo carries a large proportion of included sandstone and 
slate. Too much of the latter makes slimes and is inimical to 
successful amalgamation. 

The milling practice of Bendigo is likewise simple. Ordi- 
nary plate amalgamation is followed by a short length of 
blanket strakes succeeded by concentrators. The latter are 
I)ercussion tables of an imperfect type. The concentrates 
thus obtained go to the "pyrites works" w^here they are 
roasted and then either treated by grinding in arrastras or by 
chlorination. The Plattner or vat method has given place to 
that form of the barrel process which bears the names of 
Newberry and Vautin. Ordinary reverberatory and mechan- 
ical (both revolving cylinders and vertical shaft furnaces) 
roasters can be seen working side by side. The rate for treat- 
ment is £3, or $15 per load. A " load " of Bendigo pyrites 
weighs, on an average, 30 hundredweights. 

At the neighboring district of Ballarat much the same con- 
ditions prevail. The ores contain an increased amount of 
sulphides, due to the presence of a little more galena and 
blende, but in spite of this the percentage remains very small 
when compared to that of other mining centers in Australia 
and America. 

The stamp mills are very similar in arrangement to those 
at Bendigo. The blanket washings are usually treated in 
Berdans. Percussion tables, which are not self-discharging, 
are in general use for collecting the pyrites. Frue vanners 
have been very successfully used at the North Cornish Com- 
pany's mill, at Daylesford. The concentrates go to chlorina- 
tion works, using the barrel process. 

The three leading milling centers of Victoria, which have 
just been passed in review, have many features in common, 
but Clunes is the only one which can show a iiiill at all com- 
pletely equipped. There are no rock-breakers and no auto- 
matic ore-feeders in use either in the Bendigo or the Ballarat 
districts, though at Clunes they were introduced as early as 
1865. The absence of these very necessary parts of a com- 



A KEVIEW OF AUSTRALIAN PRACTICE. 201 

plete stamp mill is an evidence of an obstinate disregard of 
modern progress in milling methods which is as regrettable 
as it is inexplicable. 

In New Zealand, Otago, and the Thames, one in the south 
and the other in the north island, are in strong contrast. The 
southern province uses methods which originated largely from 
the experience obtained at Clunes. Gold saving by blankets 
can be seen contrasted with the more modern plate amalga- 
mation. The ore carries a noteworthy percentage of pyrites, 
which in many instances is known to lock up value. The not 
infrequent absence of any attempt at concentration is there- 
fore very noticeable. It is, however, largely explained by the 
fact that there are no works in the island which treat concen- 
trates, and as a consequence they have to be sent to Australia 
at a cost in shipping charges, etc., which is almost prohibitory. 

Going to the north island we find the reductio ad ahsurdum 
of stamp milling in the treatment of the complex gold and 
silver ores of the Thames district. Here the millstuff carries 
free gold, native silver, native arsenic, sulphides of silver and 
of antimony, arsenical pyrite, galena, blende, tellurides of 
gold and silver, and other minerals in great complexity and 
variety. This material is subjected to stamp milling, fol- 
lowed by blankets whose washings are treated by grinding 
and amalgamation in pans. The result of this process is 
that barely 60 per cent, of the values are saved, and the sub- 
sequent treatment of the tailings is almost as remunerative 
as that of the ore itself. This is an instance of the blind dis- 
regard of the first axiom of all successful milling, namely, to 
adapt the process to the ore. 

The north island of New Zealand contains many very valu- 
able depositories of the precious metals, but they are too 
often found locked up in refractory matrices. At Kuaotunu, 
Karangahake, Te Aroha, Waihi, Waiorongomai, Puhi-puhi, 
Kapanga, and other localities with picturesque native (Moari) 
names, there are ore deposits which have as yet baflled the 
best metallurgical ingenuity. In this mining region, as has 
been the case elsewhere the wide world over, large and costly 
plants have been erected before those in charge had proper 
assurance of the capability of the mine to supply the ore 
required by the mill, or of the capability of the mill to satis- 



202 STAMP MILLING OF GOLD ORES. 

factorily treat the ore. The handsome concentrating and 
leaching (cyanide) plant of the Sylvia Company, on Tararu 
Creek, is an instance of the former; and what little now 
remains of the extensive concentrating and smelting plant 
of the Te Aroha Gold and Silver Company at Waiorongomai 
is an example of the latter. 

The cyanide process has been struggling here for many 
years. It was first introduced at the Crown mine, Karanga- 
hake, and battled in vain against incompetence and ill-luck. 
At the present time it is being used with varying success at 
several milling establishments in this region, but in addition 
to the ordinary difficulties common to new leaching processes 
it has to bear the incubus of an extortionate royalty. 

Queensland is a great field for the metallurgist. At 
Gimpie, Charters Towers, Croydon, and elsewhere, the stamp 
mill extracts the bulk of the gold, but at Ravenswood and 
Mount Morga;n it has already given place to other methods. 
If the Mount Morgan mine had not contained an ore deposit 
of extraordinary richness the failure of the stamp mill might 
have proved its death blow. The gold, though free and 
enclosed in a simple quartzose gangue, was what is often 
termed "rusty." Analyses showed that though of unusual 
fineness, from 997 to 998 per thousand, it was coated with 
hydrated oxide of iron. A rough calcination, followed by 
barrel chlorination, formed a method of treatment which 
proved entirely successful. The works erected have a capac- 
ity of 1,700 tons per week. 

Of the northern gold fields Charters Towers is now easily 
first. It is one of the most successful, as regards proportion 
of expenditure to returns, of modern mining districts. The 
ore is of high grade. The mine which headed the list for 1892 
(The Victory) produced 37,752 ounces of gold from 8,775 
tons, paying £97,500 in dividends. 

The ores of Charters Towers are heavy with sulphides and 
are badly adapted to reduction by the stamp mill. However, 
here, as in Gilpin County, Colorado, such modifications of the 
ordinary methods have been made as to give much better 
results than at a first glance would be thought possible. The 
millstuff contains on an average about 5 per cent, of sulphides^ 



A REVIEW OF AUSTRALIAN PRACTICE. 203 

but this often increases to 20, 30, and occasionally even to 60 
or 70 per cent. The stamp batteries are amalgamating as 
well as crushing machines. A certain proportion, averaging 
from one-third to one-half of the total gold extracted, is 
yielded by amalgamation inside the mortars. The pulp runs 
off the outside amalgamating tables into circular concen- 
trating machines, the pyritic material being afterward sub- 
jected to grinding and amalgamation in pans. The concen- 
tration is not carried very far, viz, up to 60 or 70 per cent, of 
sulphides. Of pans there seems to be no end. At the 
Excelsior mill, for instance, I counted 50 stamps, followed by 
10 Brown & Stansfield concentrators, succeeded by 16 Blake & 
Wheeler pans and 49 Berdans. 

The treatment is expensive and only partially successfulj 
for at the close of the milling operations not more than about 
two-thirds of the value in the ore has been extracted. That 
grinding and amalgamation are inefficient in extracting the 
gold of the concentrates is proved by the fact that the chlorina- 
tion works sometimes buy the pan tailings. 

There are several chlorination plants in the district. The 
vat process is employed. Roasting costs $8 and chlorination 
$4 per ton. The grinding of the concentrates in pans can be 
shown even at the present time to be less advantageous than 
chlorination. 

Notwithstanding the high price of chemicals and fuel, 
chlorination is destined to grow in favor, especially when 
more care is given to the concentration. At the Burdekin 
mill 24 Frue vanners are at work, and by their use much 
cleaner concentrates are obtained than from the other con- 
centrators employed in the district. The time is shortly 
coming when amalgamation methods will, at Charters Towers, 
be put aside and direct concentration followed by chlorination 
succeed the existing milling practice. 

Thus we find, as the result of a brief survey of the milling 
methods of the colonies, that the gold fields of the southern 
hemisphere, like those of our own side of the world, are the 
scene of trial of many new amalgamating machines and the 
battlefield of a multitude of new processes. The clumsy old 
stamp mill and simple amalgamation treatment still hold 



204 STAMP MILLING OF GOLD ORBS. 

their own wherever gold ores are not of a complex character. 
The colonial may do well to take to heart the fact that no 
stamp mill perfect in every respect has as yet been built any- 
where, and that in his own case there is room for improve- 
ment, particularly in that arrangement of a mill which pro- 
motes automatic handling of the ore, which should make him 
regretful of the past and hopeful of the future. 



CHAPTER XIV. 
The Wear and Tear of a Mill. 

To illustrate the functions of the parts of a stamp mill we 
may employ the familiar analogy of the hammer and anvil. 
The stamp, viewed as a whole, is the hammer whose impact 
crushes the ore, and similarly the bottom of the mortar box 
may be considered the anvil upon which that crushing is 
performed. The work is, however, more particularly done 
by and upon certain small portions of the mechanism, such as 
the shoes and dies, which are therefore made so as to be 
replaceable when they break or become worn out. 

In modern milling practice the shoes and dies are made of 
a variety of kinds of iron, but it was not always so. In the 
valleys of the hills of Transylvania (Hungary) there can still 
be seen wooden stamps shod with agate falling upon a stone 
pavement lining a wooden mortar box. In the United States 
and Australia it will be found that the material of which the 
shoes and dies are made varies from chrome steel to wrought 
iron, and that the use of this or that variety of metal is a 
question to be decided quite as much by the distance from 
the foundry as by the initial cost or the excellence of the 
material. 

The shoes and dies form those parts of the mill which finally 
subjugate the hardness of the ore preparatory to delivering it 
to the agents of amalgamation ; it is on them, therefore, that 
the brunt of wear and tear necessarily falls, and, other things 
being equal, the hardest ore will cause the greatest abrasion 
of iron. But these " other things " are not always equal, and 
we therefore find that a very wide variation is effected by 
certain conditions, among which may be mentioned the state 
of division in which the ore is delivered (large or small, even 

(205) 



206 STAMP MILLING OF GOLD ORES. 

or uneven), the height that the stamp drops, the depth of the 
discharge or issue, the regularity of the feeding, and the shape 
of the mortar box itself. 

For the purpose of our inquiry into the variations in the 
wear of the shoes and dies, it will be found convenient to 
express the excellence of their service by the number of tons 
of ore crushed from the moment when they are first placed in 
position to that time when they are discarded as being worn 
out and unserviceable. The difference in weight between the 
new shoes and dies and their worn-out remnants represents 
the amount of metal consumed in the mechanical reduction 
of a certain number of tons of ore. If the remnants can be 
sold (as scrap) to a neighboring foundry the return so ob- 
tained will help, to a small extent, to diminish the initial cost. 
Figures will, however, be most expressive. In the accompa- 
nying comparative table there are given the results of the use 
of different kinds of shoes and dies in various districts and 
under varying conditions. The figures were obtained by the 
writer during the past four years and represent the practice 
of eight mining centers, four in the United States and four in 
Australasia; they give the work done and the expense in- 
curred at certain periods and under certain conditions, which, 
owing to alterations in the construction of the mills and the 
diminution of freights, are ever shifting. 

A glance at this tabulated statement will indicate the wide 
difference between the results obtained at the different mills. 
It will be our business to inquire into the reason of these 
great variations, and to endeavor to determine whether they 
are warranted by the diverse conditions which obtain in 
milling centers so wide apart. 

The wear of the shoe varies from 3.6 ounces to 21.8 ounces 
of metal per ton of ore crushed, while that of the die has a 
minimum of 3 ounces and a maximum of 7.9 ounces. In the 
matter of expense the least cost of the shoe is at the rata of 
2.02 cents and the greatest 7.64 cents per ton of ore, the 
minimum cost of the die is at the rate of 0.71 cent and the 
maximum 5.6 cents, while the combined cost under this head 
varies from 4.06 cents to 13.14 cents for every ton of ore 
crushed. 

Brief reference to each milling center quoted will be of 



THB WEAR AND TEAR OF A MILL. 207 

service in explaining some of these differences. In Gilpin 
County, the oldest established mining center of the State of 
Colorado, we find that cast iron is the metal employed. 
Chrome steel shoes and dies, manufactured at Brooklyn, N. 
Y., have been tried, but the millmen of Black Hawk prefer 
the product of the local foundries. It is mainly a question 
of economy. Though the steel wears in the ratio of only 9.3 
ounces per ton of ore as compared to 15.7 ounces of cast iron, 
yet the former costs twice as much as the latter, and therefore 
the resulting expense is in the proportion of 7.15 cents to 5.95 
cents. Here, as is usually the case, the scrap iron is salable 
at 1 cent per pound, while the steel remnants are worthless. 
In the actual working of the mill it has been found that cast 
iron wears more evenly than steel, the latter tending to 
develop an irregular surface ; this (called " cupping" by the 
millmen) diminishes the crushing surface and increases the 
Tibration of the mechanism of the stamps. 

The wear in this district is excessive, though the ore is of 
less than ordinary hardness. This is due in part to the 
extremely long drop prevailing in the mills, namely, from 16 
to 20 inches, but it is also caused by the absence of rock- 
breakers and automatic feeders. 

At Grass Valley, in Nevada County, Cal., the ore is 
extremely hard. It is composed of white quartz and a vary- 
ing but very large proportion of the syenitic country rock. 
The metal of the shoes is chrome steel, which comes from 
New York, while the dies are cast at the local foundry and 
contain a fifth part of steel, being the remnants of worn-out 
shoes. It has been found, at the North Star mill, that the 
use of cast-iron dies with steel shoes materially adds to the 
life of the shoes, and is much better than the use of the same 
metal in both shoes and dies. At the Empire mill both cast 
iron and steel dies are in use. There does not appear to be 
any marked difference in their manner of wear, and the steel 
does not exhibit "cupping." The seeming contradictory 
feature of this experience, as against that of Gilpin County, 
Col., is to be explained by the fact that the more rapid drop 
of the California mill, accompanied as it is by a turn of the 
stamp, tends to equalize the wear and to maintain an even 
surface better than the very slow drop of the Colorado mill. 



208 STAMP MILLING OF GOLD ORES. 

At the Idaho mill chrome steel from Brooklyn has been found 
to give much longer service than steel shoes and dies manu- 
factured in California. When the chrome steel and cast-iron 
dies do not show much difference in the cost per ton of ore 
crushed, it is found that the millmen prefer the former 
because, though more costly, they last longer and therefore 
require less frequent replacement. 

At Angels Camp, in Calaveras County, Cal., the conditions 
are very favorable to a minimum wear. At the time of my 
last visit the height of drop was 6 to 6 inches and the stamps 
dropped 95 times per minute. Since then the principal plant, 
the Utica mill, has been enlarged and the methods of milling 
slightly modified. The Stickles mill has also come under the 
Utica management. The ore of this district is particularly 
soft ; the quartz occurs in small seams forming very wide 
lodes, in which there is more slate than quartz. The millstuff 
is readily crushed by the stamps. Experience has taught the 
local millmen that steel shoes and cast-iron dies give better 
results than shoes and dies of similar metal. The difference 
between the wear of steel on cast iron, and of cast iron upon 
cast iron, is found to be very small, while the cost per ton 
varies by a fraction of a cent only. 

At Mammoth, in Pinal County, Ariz., the conditions are very 
different from those with which we have had to deal in any 
of the three milling centers previously noted. The nearest 
railway is 52 miles distant, and there is no foundry able to 
provide cheap castings. Chrome steel shoes and dies are pre- 
ferred because the freight on cast iron is so high (from 
Denver, 2^ cents per pound) that the extra length of service 
of the steel more than compensates for the increased first cost. 
Steel costs 11 cents per pound, delivered, as against about 7 
cents for cast iron. Here there is no doubt as to the fact that 
the greater length of service of chrome steel much more than 
compensates for its slightly higher cost. 

The ore of this district is not very hard and the wear must 
be considered excessive. It is due to some extent to the ab- 
sence of sizing-bars (grizzlies) in the mill and the consequent 
unevenness of the material delivered to the stamps, and it is 
also in part caused by the small but varying depth of discharge 
which has a minimum of 1^ and a maximum of 6 inches. 



THB WSAB AMD TEAR OF A MILL. 





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THB WEAR AND TEAR OP A MILL. 211 

Going to the southern hemisphere, we find the wear and tear 
of shoes and dies is very much in excess of that to be noted 
in the United States. At Bendigo both shoes and dies are 
furnished by the local foundries. The former are. invariably 
made of cast iron and the latter of wrought iron. The local 
prices are £12 per ton for both " hard " metal shoes and "goft" 
metal dies. Old cast iron brings £4 per ton and wrought iron 
scrap iE^li P^r ton. Steel, when imported from England, 
costs £30 per ton, and while it has been found to give much 
longer service than the local castings, its high price renders 
its use prohibitive. The shoes generally weigh from 180 to 
195 pounds and are usually 9 inches high by 9^ to 10 inches 
in diameter. When worn out they weigh from 35 to 42 
pounds. They give very poor service, 16 to 25 ounces of iron 
being worn away for every ton of ore crushed. Neither do 
they wear down evenly, but exhibit an irregular surface which 
much impairs their usefulness. The dies weigh from 80 to 
110 pounds, their depth varies from 3^ to 4^ inches. When 
worn out they weigh from 20 to 30 pounds. They give excel- 
lent service, wearing slowly and evenly. The loss of iron per 
ton of ore crushed varies from 3^ to 13^ ounces. 

Notwithstanding the excessive wear of the shoes, yet by 
reason of the excellent service given by the dies and because 
of the low first cost of the metal of both shoes and dies, the 
total cost is only about 4^ cents per ton of ore, a figure which 
compares well with the same item of expense at the American 
mills. 

At the Thames district, in New Zealand, the excessive wear 
of the shoes is again marked and is again due to the absence 
of rock-breakers and automatic ore-feeders, causing the deliv- 
ering of irregularly broken millstuff at a variable rate de- 
pendent upon the caprice of a combination of boy, shovel, 
and sledge hammer. 

Both shoes and dies are made of cast iron obtained at a 
local foundry. They differ in that the former is, and the 
latter is not, chilled. The price is £13 per ton. Old scrap is 
taken at £5 per ton. 

The shoes weigh from 168 to 215 pounds and vary in depth 
from 9 to 10 inches. When worn out they weigh from 30 to 
50 pounds. The wear is equivalent to from 6 to 16 ounces of 



212 STAMP MILLING OF GOLD OBES. 

iron per ton of ore crushed, the minimum rate being only 
obtained with soft surface ores. 

The dies when new weigh from 80 to 116 pounds ; they are 
usually octagonal and they have a thickness varying from 3^ 
to 5 inches. When worn out they weigh from 35 to 45 pounds* 
The wear varies from 5^ to 8| ounces per ton of ore crushed. 

The service given by both shoe and die is poor, and is 
largely due to the very bad, irregular feeding which is common 
to the mills of the district. The very variable hardness of 
the ore and the little care taken to maintain a constant depth 
of discharge are also factors in the production of this exces- 
sive waste of metal. 

At Clunes, as at Bendigo, cast iron is worked against 
wrought iron. The shoes weigh 192 to 198 pounds when new, 
and 45 to 60 pounds when worn out. They do not wear so 
evenly nor last so long as the dies, which weigh 130 to 140 
pounds when new, and from 25 to 35 pounds when discarded. 

The ore treated at the mills of this district is very nearly 
clean quartz, and though it is readily broken, its ultimate 
pulverization produces a heavy wear and tear. The South 
Clunes United mill has, unlike the other two principal plants 
(now idle), no rock-breaker, and therefore the wear of the 
shoe is very excessive. The low cost of the castings, however^ 
makes the final cost far from high, since it amounts to only 
5| cents per ton of ore treated. 

At Harrietville, also in the colony of Victoria, the shoea 
and dies are made of the same material, viz, fagoted white 
iron. It is obtained from a Melbourne foundry. The cost m 
£16 per ton. The castings are unsalable at Harrietville. 

The shoes are 9 inches high by 9^ inches in diameter ; they 
weigh, when new, 172 pounds, and when worn out, 38 pounds. 
They give good service and retain a fairly even surface. 

The dies are octagonal and 4 inches deep. They weigh 84 
pounds when new, and 37 pounds when discarded. Their time 
ot service is less uniform than that of the shoes, and they do 
not retain so even a surface. The metal of which they are 
made should be, but is not, more tough than that of the shoes. 
The total cost per ton of ore amounts to 4| cents. 

We have now passed in review the eight districts whose 
figures are given in the comparative table. Can anything be 



THE WBAR AND TBAB OP A MILL. 218 

learned from the comparison or are we to decide that the 
results are too contradictory to warrant any generalizations ? 
We shall, I believe, find that there is more harmony in the 
evidence of these figures than would at first sight appear. 

Take, for instance, the fact, which is readily apparent, that 
the wear of the dies does not vary in an equal degree with 
that of the shoes. The minimum to the maximum is in the 
former as 1 to 2.6, but in the latter it is as 1 to 5*9. The 
explanation is to be sought for by referring to the analogy of 
the hammer and anvil. The die, which is the anvil, is pro- 
tected by the ore which lies upon it, and the brunt of hard 
work falls upon the shoe, which is unprotected. 

The ore upon the die is a cushion, and the more constant the 
thickness of that cushion the less the wear of the die. Here 
is where bad feeding does its evil work. The regular supply 
of ore particles of even dimensions is never obtained by the 
use of the sledge hammer and shovel ; the absence of sizing- 
bars (grizzlies) and of automatic feeders is a potent cause in 
diminishing the time of service of the dies. In this connec- 
tion it may be pointed out that the depth of discharge — ^the 
distance from the bottom of the screen to the top of the die — 
is a factor whose importance is universally underestimated in 
stamp milling. In many mills, particularly in Australia, it 
will vary in the ratio of 1 to 2, or even 1 to 3^ ; such variation 
must necessarily assist in impairing the regular working of 
the mill and cause a heavy waste of iron, because under such 
changing conditions it is doubly difficult to regulate the thick- 
ness of the ore upon the die. 

Experienced millmen always maintain that the use of a 
rock-breaker diminishes the wear of the shoes and dies. Is 
this confirmed by the tabulated figures ? If you examine the 
results obtained under varying conditions, you will find that 
every milling center which does not use the rock-breaker, pre- 
ferring the simple barbarity of sledge-hammer treatment, has 
an excessive wear, which is most noteworthy in respect to the 
shoe. The effect upon the die is not marked, because, as 
explained above, the cushion of ore upon it serves as a pro- 
tection, equalizing the wear and tear. 

It will be noticed that the mills of Gilpin County, the 
Thames, Bendigo, Olunes, and Harrietville, though working 



214 STAMP MILLING OF QOhD ORES. 

under a variety of conditions and crushing ore of very differ- 
ent hardness, have each of them a wear of iron exceeding 10 
ounces per ton of ore crushed, while the rock-breaker dis- 
tricts, — Grass Valley, Angels Camp, and Mammoth, — also 
working with ores which have a very different character, from 
very soft to extremely hard, show none of them a loss of iron 
exceeding 7^ ounces per ton of ore* In the cases of Grass 
Valley and Bendigo, the contrast is very marked, since while 
the millstuff of the California district is extremely hard, yet 
the liberal use of rock-breakers renders the wear only one- 
third that of the Australian center. 

These results are in accord with common everyday experi* 
ence. It is the hammer that must suffer the brunt of the 
unnecessary wear entailed by the attempt to crush particles 
of stone of unequal size. One moment the stamp falls upon 
particles of ore fine as dust, but the next it is dancing upon a 
large stone which it does not break at the first blow, doing 
useless work and having its iron unnecessarily abraded and 
chipped. 

The low price of castings in the milling centers of the 
antipodes accentuates the high prices which still obtain in the 
mining camps of the United States. These high prices 
seriously handicap the economic success of mining and milling 
in the Great West ; they are in many instances out of date 
and out of keeping with the diminution in freights and the 
cheapening, all over the world, of everyday commodities. 

The great diversity of metal used in the shoes and dies of 
the various mills is due to a rule-of-thumb policy. The 
analogy of the hammer and anvil shows that the metal of the 
anvil should be, and is, softer and tougher than that of the 
hammer. It should be so in the case of the shoe and die. 
The best results are to be obtained not so much by the 
employment of the hardest material, as by having the die 
made of metal more tough,* less brittle, than that of the shoe. 
Thus steel and cast iron, chilled and unchilled iron, cast and 
wrought iron, are combinations which generally give good 
service. The excellent work done by wrought-iron dies at 
Clunes and Bendigo should teach a lesson to those who are 



*The use of chrome steel shoes with manganese steel dies is to be highly com- 
mended. 



THE WEAR AND TBAB OF A MILL. 215 

seeking to obtain a metal of excessive hardness for the 
manufacture of dies. 

In summing up we find, as common sense and ordinary 
experience would suggest, that in the matter of the shoes it 
is the use or otherwise of the rock-breaker (with grizzly and 
self-feeder) which will most affect their wear, and in respect 
to the die the desideratum is a metal which shall be less hard 
and more tough than that of the shoe which falls upon it. 



CHAPTER XV. 
Thb Flouking op Mercury. 

Millmen rarely have a clear idea of the reasons which 
cause the mercury used in a stamp mill to refuse to amalga- 
mate with the gold particles and to be carried away in the 
tailings. "Sickening" and ''flouring" are the terms used, 
but with a confusing carelessness. Thus a recent writer 
speaks of the loss of mercury in a certain mill as being 
'" greatly increased by bunches of ore carrying oxide of man- 
ganese, which causes the mercury to ' flour.' " 

Although the causes which produce what is known as 
"floured" mercury may not be accurately or completely 
known, I nevertheless feel that the writer, in the instance 
above quoted, like others in other cases which have come 
under my notice, has misconceived the meaning of the term 
which he uses. In the same way the reactions which produce 
the "sickening" of mercury are but rarely, and then only 
vaguely, understood. 

The subdivision of mercury into minute globules is " flour- 
ing." The term has, of course, been chosen because the sub- 
division referred to is carried so far as to cause the mercury 
to resemble a whitish-gray powder. When mercury is sub- 
divided the globules are very apt to become coated with a film 
of any foreign substance which may be present, the effect of 
which is to prevent them from coalescing or reuniting. Thus, 
therefore, "sickening" follows "flouring" and renders the 
latter condition permanent by the preventing of the reunion 
of the subdivided particles of mercury. " Flouring " by itself 
is not necessarily injurious, but rather favorable to amalga- 
mation, if only the minute globules of mercury alloy them- 
selves with the gold in the ore before they become coated 

(216) 



THE FLOURING OP MERCURY. 217 

with foreign matter, such as will hinder amalgamation and 
at the same time render them readily transported by the water 
running over the plates. 

" Flouring " always occurs in a stamp mill to a varying 
extent ; it is due to the violent agitation of the pulp, which 
takes place under the stamps, and to the cutting of the 
mercury (fed into the battery) by sharp grains of ore. The 
** sickening " which may then ensue, and generally does, more 
or less, is due to causes either mechanical or chemical, or 
both. 

Thus, for instance, the globules of mercury may become 
coated with grease from lubricants used in the mine or mill ; 
they may become covered with a film of some unctuous min- 
eral, such as talcose clay. Grease dropping from the guides 
often does mischief. To prevent it as much as possible I would 
recommend the use of a mixture of one quart of plumbago in 
one gallon of molasses. Similarly, the varnish on the surface 
of new screens is injurious. It is well, therefore, before 
putting them in position, to soak them in a hot solution of 
concentrated lye. Where, on the other hand, the ore itself 
carries an excess of clay, the best thing to do is to mix some 
granular millstuff with the ore fed into the battery. In some 
mining districts the gold-bearing quartz occurs in veins tra- 
versing alternations of slate or schist with sandstone or some 
other granular rock. In such cases it only necessitates a little 
care in feeding the mill with a judicious mixture of the granu- 
lar and the softer, more clayey, material. 

The use of hot water in the stamp mill, to prevent freezing 
in a cold climate, or to aid amalgamation in a warm region, 
often leads to the employment of condenser water. This is a 
serious mistake, because such water usually carries the mill- 
man's worst enemy — grease of some kind. I remember well 
visiting a mill at Ballarat and being surprised to see steam 
escaping (it was summer time) from the water running 
over the plates. The millman told me he was using con- 
denser water, and when I suggested that it was a danger- 
ous aid to amalgamation he demurred. A few days previous 
I had examined the launder conveying the condenser water at 
a neighboring mill, and had found the sides and bottom cov- 
ered with a slimy ooze which could not but be deadly to the 



218 STAMP MILLING OF GOLD ORBS. 

eflSciency of mercury in a mill. I asked him what amount of 
mercury the mill consumed, expecting to be able to show him 
triumphantly that he was incurring an excessive loss of that 
volatile metal. But, to my discomfiture, I found that the 
consumption per ton of ore crushed was slightly less than at 
his neighbor's plant where condenser water was not used. 
Somebody evidently was mistaken. On further skirmishing 
about the mill I found that it was the practice of the foreman 
to add 5 pounds of quicklime to each battery of 5 stamps every 
24 hours. Although intended primarily to neutralize any 
excessive acidity in the battery water, the lime, as an alkali, 
was also a solvent* for the grease present in the condenser 
water and was serving as an effective antidote. 

Many of the patent amalgamating machines daily invented 
have as their underlying principle the idea that the more you 
bring the gold into frequent contact with the mercury, the 
more complete the amalgamation will be. The inventor 
usually sets out to obtain such frequency of contact by com- 
pelling the ore to pass through a bath of mercury, or else he 
turns the mercury into a spray, or mixes the ore and the mer- 
cury violently together, or has some other scheme of which 
the essential feature is that the mercury is cut up and sub- 
divided into particles of more or less minuteness. Such ideas 
would be well enough were it not for the fact that it is much 
easier to cause mercury to subdivide than to recoalesce, since 
of the large amount of foreign substances present in a pul- 
verized form there are always some which at once proceed to 
coat the globules of mercury, preventing their reunion and 
rendering them easy of transport by water. A simple in- 
stance can be cited in the case of the amalgamating barrels 
used in many mills for the treatment of blanketings, pan 
tailings, skimmings, etc. It is usual to add pieces of iron, 
such as old bolts, fragments of shoes and dies, etc., in order to 
promote a grinding action. Very often the speed of the 
barrel is too rapid or the quantity of scrap iron is too great, 
and if you take a pan and wash a few handfuls of the waste 
ejected from the barrel you will find a large amount of floured 
mercury. 

* Through saponification. 



THE FLOURING OF MERCURY. 219 

"Sickening," again, may be due to chemical causes, the 
mercury probably becoming in some instances covered with an 
oxide or even a sulphide of itself. In the case quoted at the 
beginning of this chapter the cause may have been partly 
chemical, the oxide of manganese giving up some of its oxygen 
to the mercury, or partly mechanical, if the oxide of manga- 
nese was in its common pulverulent condition such as would 
render it easy to coat the globules of mercury. 

Cyanide of potassium, sodium amalgam, and other reducing 
agents, induce a reunion of the particles of mercury probably 
because of the vigorous action of hydrogen in a nascent state 
liberated while in contact with the mercury. An electric 
current similarly helps to counteract sickening. Again, I 
might quote the use of plates made of Muntz metal (such as 
are commonly employed in certain parts of New Zealand), 
which, being an imperfect alloy of copper and zinc, sets up a 
feeble local galvanic action, electrolizing the water, liberating 
the hydrogen, which in turn reduces any oxides which may be 
coating the quicksilver. 

Thus, therefore, I would define "flouring "as the minute 
subdivision of mercury by mechanical causes, and "sickening" 
as the rendering of such a condition permanent by the inter- 
vention of a coating of some foreign substance, effectually pre- 
venting the coalescence or reunion of the globules previously 
formed. 



CHAPTER XVI. 
Comparisons. 



The material of this chapter has already appeared in VoL 
IV of The Mineral Industry* smidi as it summarizes the infor- 
mation given in the preceding pages of this book, it is now, 
by the kind permission of the publishers, reproduced. 

In the accompanying tabulated statement an endeavor has 
been made to exhibit the distinctive features of the milling 
practice of representative districts in several countries. It 
is very difficult to arrive at "average" figures, and it is not 
always easy to determine which mills are typical and which 
are abnormal in their methods. Nevertheless it is hoped that 
the data given, which are founded on a personal knowledge 
of each locality, will serve as a trustworthy basis of com- 
parison. 



District. 


Labor. 


Shoes and 
dies. 


Water. 


Fuel. 


Supplies, 
etc. 


Total. 


filack Hills. . . . 

Gilpin 

Grass Valley . . 
Amador 


16 
42 
82 
20 


2 


20 

' ' 81 * ' 

17 


19 
10 


14 
8 


70 
76 
80 
46 



Even the well informed will be surprised at the costs of 
treatment given in the second column of the table. That the 
large establishments of the Homestake Company (in South 
Dakota) should not be able to work with any considerable 
diminution of the expense incurred by the smaller and much 
less completely equipped plants of Black Hawk (in Colorado), 
in spite of a rapidity of crushing four times as great, is a fact 
demanding an explanation. That the mills of Grass Valley 
(in California), which are the model plants of this country, 
should treat their ores at a cost greater than either of the 
a bove-quoted regions seems also odd. 

* Published annually by the Scientific Publishing Co Jipany of New York. 

(220) 



C0MPABI60NS. 221 

An analysis of the costs of milling in the four representa- 
tive American districts will indicate the causes underlying 
these anomalous results. These figure are per ton of ore and 
are given in cents. 

The item of labor is dependent upon the rate of crushing 
and the automatism of the mill. In the latter regard the 
Gilpin County district is the only one exhibiting defects, for 
the mills of the other three are uniformly provided with rock- 
breakers, self-feeders, and other appliances inseparable from 
an economic handling of the ore. And although an inadequate 
equipment of labor-saving machinery still remains a char- 
acteristic 01 the old plants at Black Hawk (in Gilpin), it is 
well to add that the new mills erected in other parts of Colo- 
rado during recent years have been quite up-to-date in this 
respect. This is not true of the Australian colonies, however, 
for there even new mills have been designed on obsolete lines, 
and a plant recently built at Bendigo perpetuates the ancient 
barbarism of breaking and feeding the ore with hammer and 
shovel. 

The labor cost in the Black Hills is less than it is in Cali- 
fornia, notwithstanding a higher rate of wages,* because the 
crushing capacity of the batteries is nearly double. This is 
largely due to the employment of a mortar of a most excel- 
lent design, promoting amalgamation while hastening pul- 
verization. The labor cost at Grass Valley is higher than that 
of its neighbor, Amador, because the crushing rate is in the 
proportion of 2 to 8, itself a consequence of a decidedly harder 
ore. The maximum cost which distinguishes Gilpin is trace- 
able to a low rate of crushing accompanying the absence of 
labor-saving devices. 

The cost in shoes and dies exhibits a very striking variation 
because the price of the iron or steel used is dependent upon 
purely local conditions, such as the distance from the foundry. 
The wear, as measured in ounces of metal consumed per ton 
of ore crushed, is as follows : Black Hills, 13 ; Gilpin, 14^ ; 
Grass Valley, 16; Amador, 15. The Homestake Company 
makes its own dies and buys the shoes in large lots, hence the 
cost is relatively small. The Grass Valley mills use steel for 

* Ordinary laborers get |8 per shift. In Colorado and California the rate is |2.5(X 
The skilled workmen are paid proportionately. 



222 



STAMP MILLING OP GOLD ORBS. 



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COMPARISONS. 223 

their shoes and local castings for their dies. The price of the 
former is high, since they come across the continent — from 
Brooklyn. Furthermore, the ore crushed in this district is 
very hard. The Amador mills use shoes and dies made at 
Sutter Creek, in the same county, and sell back the remnants. 
The Gilpin County plants are provided with local castings 
and also resell the scrap. The greater softness of the ore of 
this region is offset by the high drop which is conducive to 
excessive wear. 

The cost of water falls heavily upon the establishments in 
the Black Hi] Is, because they pay a subsidiary company for 
that which they use in the batteries, and the item of fuel is 
likewise a serious one, because their motive power comes from 
steam obtained from the burning of wood also provided by a 
subsidiary company. The fuel and water cost amounts to 39 
<;ents, or more than half the entire milling cost. The Cali- 
fornian mills pay for their water at the rates varying from 18 
to 20 cents per miner's inch (of 1.574 cubic feet). The 
Black Hawk mills get their water free from a dirty creek 
laden v/ith partially oxidized pyrites whose acid corrodes the 
screens. In winter a diminished water-power is supplemented 
by steam. 

The cost of supplies, mercury, chemicals, lubricants, etc., 
depends largely upon the distance from a large manufacturing 
town. It is heaviest in the Black Hills and least in Colorado. 

These explanations will now have rendered intelligible the 
resulting totals, which prove that the mills of Amador do 
much the cheapest work, and in this respect they are typical 
of Calif ornian practice, in Calaveras, Tuolumne, Mariposa, 
Plumas, and other representative districts, more so than 
Grass Valley, whose millstuff has a quite exceptional hard- 
ness. Widening our survey to foreign lands, we note that the 
three chief gold-mining centers of Victoria exhibit closely 
approximating averages, while Charters Towers has a high 
€ost because its ores are not amenable to simple treatment, 
but require an expensive supplement of pans and settlers. 
The New Zealand districts are similarly related, Otago's ores 
I)eing simple and docile and those of the Thames complex and 
refractory. Thus because of complete equipment and cheap 
motive power the mills of California still retain their pride 



224 



STAMP MILLING OF GOLD ORBS. 



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COMPARISONS. 225 

of place as affording the least expensive method of reduction 
yet devised by man for the obtaining of gold from its ores. 

The number of stamps in a mill is ordinarily, but not 
always, dependent upon the output of the mine behind it. 
Where the plant is erected upon a flat mill site, 80 stamps, 
divided into two rows, back to back, makes a very convenient 
and economical arrangement. Where a graded mill site, 
upon a hill slope, is chosen, 40 to 60 stamps is a good number. 
Up to this size there is no proportionate increase in labor 
costs, but a further enlargement requires a notable augmenta 
tion of the force employed. Tj avoid losses by fire or flood it 
is, moreover, advisable not to have too many stamps under 
one roof. 

The weight of stamps varies from 500 pounds to twice as 
much. Only prospecting plants are now provided with 
stamps of less than 500 pounds, although some old mills are 
still at work with stamps ot 400 to 450 pounds. The light 
weights in use in the old-fashioned Colorado mill are rendered 
as effective as the heavier type because of their long drop. 
The most desirable weight for given ores is dependent, much 
more than is usually supposed, upon the attainment of con- 
ditions favorable to amalgamation. Thus the light stamp (of 
Gilpin) is the consequence of the long drop, and the long drop 
is necessary in order to obtain the interval required to permit 
of the action of gravity in causing the minute particles of 
gold to separate from the pyrites and settle upon the amal- 
gamated plates placed within the mortar. Where the gold 
particles readily detach themselves from the quartz or other 
incasing mineral it is not necessary to emphasize this feature, 
and heavy quick-dropping stamps are desirable because their 
greater crushing capacity diminishes the cost per ton. 

On the other hand, while the added weight gives greater 
crushing force to the stamp a practicable limit is soon reached, 
because the pulverization becomes too rapid for the amalga- 
mation, the ore being reduced so fast that there is not suffi- 
cient opportunity given for the gold to be arrested by the 
mercury on the plates. Steam stamps have for this reason 
proved unsuitable for gold milling, as was proved by the 
experiments made by the Homestake Company, described in 
Chapter VI. The Alaska- Treadwell Company, among others, 



226 STAMP MILLING OF GOLD ORES. 

carried out a series of experiments and found that a weight of 
over 1000 pounds made the mill a rapid pulverizer but a poor 
amalgamator. 

The heaviest stamps are usually given to mills designed 
with iron frames. And, in parentheses, it may be asked why 
iron frames are not in more general use. At Bendigo and 
Ballarat such batteries have been pounding away for 30 
years past and are yet working sweetly. The supposed weak- 
ening of the iron because of the excessive vibration is a 
bugbear feared by many. A certain simple detail of con- 
struction readily prevents any injurious results. Such mills, 
it may be added, are particularly adapted to the conditions 
obtaining in some of our Western mining regions, where 
suitable lumber is becoming scarce at a rate proportioned 
to the cheapening of iron and steel following upon lessened 
railroad freights and multiplication of machine shops. 

The height of the drop is necessarily proportioned to its 
rapidity. A high drop needs a long cam, and the endeavor 
to get speed out of such a combination invariably leads to 
breakages. Similarly a low drop permits the use of short- 
armed cams, which lessen the leverage and decrease the strain, 
allowing of greater speed. A very heavy stamp is not needed 
with a long drop, nor is it practicable. The fastest work is done 
by a 750-pound stamp dropping 6^ inches 95 times (with a 
maximum of 105) per minute ; the slowest arrangement is the 
old Colorado mill with its 550-pound stamp falling a height of 
17 inches (and sometimes as much as 20) at the rate of 30 times 
(occasionally as low as 25) per minute. The divergence of 
practice in this respect is most assuredly wide enough and 
furnishes material for a very interesting study.* 
^"^^rl aI '^h® depth of discharge, that is, the distance from the level 

f of the issue (the bottom of the screen) to the top of the die, 

is a factor whose importance in stamp milling is generally 
underrated. The deepest discharge is from 14 to 16 inches ; 
the shallowest from nil to 5, with an average of 2 inches. 
The maximum depth gives conditions which compel slow 
crushing and a pulverization more minute than the fineness 
of the screen indicates, because the pulsation of the water 



/, 



.. -^ 



It' 



* See Chapter II ; also the Transactions of the American Institute of Mining JEngi- 
neerSt Vol. XXIII, p. 187, et seq.f also p. 569, et seq. 



COMPARISONS. 227 

inside the mortar is weakened and the force of the issue 
impaired. The minimum discharge gives only a thin cushion 
of water between the ore upon the die and the descending 
stamp; it causes a violent splash and a forceful issue. In 
the Australian mills there is but rarely any serious effort 
made to obtain a uniformity in this respect, so that the 
discharge will usually vary from 1 inch with the new dies to 
5 inches as they become worn out. Obviously a difference 
of 3 or 4 inches causes more variation in the case of a shallow 
issue than would result from the wearing down of the dies in 
a deep mortar where the depth of discharge is so great that 
the increase is only a small fraction of the total. Hence this 
factor causes more irregularity in the operation of the 
Colonial and Californian mills than in those of Colorado or 
Dakota. 

In two districts, the Black Hills and Amador, the height of 
the drop is less than the depth of discharge, which means that 
in these instances the stamp is never lifted entirely out of the 
water in the battery. It is doubtful whether this condition 
seriously impairs the crushing capacity of the mill. It pro- 
motes a regular pulsation of* the water and pulp such as con- 
duces to regularity of discharge, itself nearly synonymor.s 
with rapidity, since not more than a certain amount of pulp 
can go through the screen apertures at any given time, no 
matter how violent the splash of the water against them. It 
does also, I believe, favor conditions helpful to amalgamation, 
because as against a very violent splash it permits of the 
settling of the gold particles and minimizes scouring action 
on the inside plates. 

The rate of crushing should not be surprising in the light of 
the consideration of the different factors which regulate it. 
One factor, namely, the character of the ore, its composition 
and its quality, is given in the last column of the table. The 
possession of very large bodies of extremely low-grade ore is 
sufficient explanation for the hurried treatment which 
characterizes the methods of the companies in the Black 
Hills. The average Californian and Australian mills treat an 
equal tonnage, and the millstuff supplied by the mines comes 
from ore depositories of a parallel magnitude. New 
Zealand comes next, and our old friend, the Gilpin County 



228 STAMP MILLING OF GOLD ORES. 

mill, is last. And this, too, is not strange, for the mines of its 
particular habitat are restricted in the extent of their workings 
and always carry a certain proportion of ore sufficiently rich 
to warrant direct shipment to the valley smelter. Moreover, 
it treats an ore which in most districts would be considered 
hopelessly refractory and is rendered amenable to successful 
reduction under the stamps by a modification of the ordinary 
process which, anomalous as it often seems to the uncompre- 
hending visitor from other districts, is yet remarkably adapted 
to the conditions of the case. 
In the matter of screens there is a perplexing confusion, 
^^{Ai owing to the loose nomenclature and divergent types of the 
apparatus employed. The coarsest crushing is done at Clunes, 
where the practice is to use thick copper plate perforated with 
only 100 holes per square inch. This lasts during the passage 
of 360 tons of ore. Since no mercury is put into the mortars 
of this district, the use of copper is not objectionable on the 
ground of its liability to amalgamation. Moreover, the coarse 
sizing of the pulp is permissible on account of the extremely 
simple character of the millstuff, the gold of which is readily 
loosened from the fractures and cavities in the quartz. Fine 
crushing is neither needed nor desirable in preparing such 
an ore for the blanket tables and wells which form the char- 
acteristic gold-saving apparatus of this old mining center.* 

The finest pulverization is accomplished by the mills of 
Gilpin because the intimate association of the gold and 
pyrites requires such treatment in order to compel a separa- 
tion. The screens of this district can hardly be considered in 
the nature of a sizing apparatus ; they are made of iron plate 
punched with alternate burr slots of such a number and size 
that the chances of a particle of pulp, adequately pulverized, 
effecting an exit are as 1 in 34. This results in a degree of 
crushing far in excess of that indicated by the mesh of the 
screen, because particles once reduced so as to permit of their 
passage through the openings are thrown back for further 
comminution. The capacity of the mill suffers, but condi- 
tions are obtained suited to the separation of the fine gold 
from its envelopment of pyrites.f 

* For details of the Clunes practice reference should be made to Chapter VII. 
f See also Transactions American Institute Mining EngineerSf Vol. XXIII, p. 18d. 
et seq. 



COMPARISONS. 229 

It is a curious fact that in none of the districts quoted in 
the table are wire screens in the ascendant at the present 
time. In some instances they have been replaced, within 
recent years, by punched plate. This seems to me a curious 
and unfortunate retrogression in milling practice. Neverthe- 
less woven wire cloth is destined to supersede punched or 
perforated iron plate. Wire screens have apertures approxi- 
mately equal to the thickness of the wires of which they are 
made, and therefore offer equal chances for the discharge or 
retention of such particles of pulp as have been sufficiently 
crushed, as the discharge area depends upon the relation 
between the mesh and the thickness of the wire. With a 27 
mesh, and a 33 B. W. G. wire they are equal. They give 
conditions promoting uniformity of pulverization and a high 
crushing capacity. A recent test made at the Mammoth Mill, 
in Pinal County, Ariz., showed that as against a No. 6 angle- 
slot screen and an equivalent wire cloth of 24 mesh and No. 
26 wire, the latter crushed 20 per cent. more. They are more 
liable to be choked by the pulp and weakened by wood 
chips from the mine timbers, and therefore in well-managed 
mills we find at least two sets for each battery, so as to permit 
of a regular substitution pending cleaning or repairing. This 
is urged against their usage, but surely it is a very foolish 
economy to sacrifice the proper operation of the mill in the 
effort to diminish this item of expenditure. The ordinary 
expense in screens per ton of ore is about 1 cent, so that even 
the doubling of this is no matter so long as conditions for 
good work are attained. 

The life of the screens depends upon the hardness of the 
ore, the acidity of the battery water, the attention given to 
them, etc., but more than these it is chiefly regulated by the 
length of time which the superintendent considers it desir- 
able to retain them in service. A screen will often actually 
last without breakage (at the Homestake, for instance) about 
20 days, but its openings become so enlarged by abrasion as 
to permit of coarse crushing to an extent incompatible with 
successful extraction; therefore it is the practice of good 
millmen to throw it out after an average, say, of 7 days. This 
is sensible. False notions of economy cause some men to 
lose ounces of gold in the effort to save a few cents. In Gilpin 



280 STAMP MILLING OF GOLD ORES. 

County the enlargement of the screen openings is allowed to 
go further than is customary elsewhere, and when the screen 
has had a good service in that part of the mill which is crush- 
ing ore from the company's mine, it is transferred for use to 
the section employed in treating custom ores. 

The Thames and Charters Towers districts exhibit the brief- 
est service, because the ores of these regions are rich in the 
sulphides of base metals whose partial oxidization gives an 
acidity to the water, causing a corrosion of the iron of the 
screens. The mills of Black Hawk exhibit a remarkable 
difference in wear of screens, with a rapid diminution of ser- 
vice as the mills succeed one another down stream, so that 
from a maximum of three months the average life of a screen 
dwindles to two weeks. This is due to the fact that the water 
which has been used in the batteries of one plant is reused in 
its next neighbor, with a consequent constant increase in 
acidity due to the decomposition of iron and copper sulphides. 
The large percentage of these is indicated in the next 
column. In some instances the values saved by the mill are 
so largely in the pyrites collected on the shaking tables that 
the milling operation becomes more of a concentration than 
an amalgation process. At Charters Towers the proportion of 
sulphides is also a heavy one, and the preliminary battery 
treatment is supplemented by regrinding in pans, followed 
finally by the chlorination or cyanidation of the richest por- 
tion of the concentrates. At the Thames the ores are equally 
loaded with refractory sulphides, but in that locality no ade- 
quate effort is made to prevent them from escaping with the 
tailings into the sea. The saving of gold remaining in close 
association with the sulphides is also very inadequately 
attempted by the Homestake mills. The plant as yet erected 
for this purpose can only be considered an incompletely 
planned experiment.* The ores carry from 2 to 5 per cent, 
of pyrite, pyrrholite, and mispickel. 

The principal Australian and Californian goid-milling 
centers have ores giving an approximately equal percentage 
of concentrates. Thus we gradate from heavy sulphide ores 
of a very refractory character, for whose treatment stamp 
milling is only desirable on account of compulsory local con- 

* See also Chapter VI. 



COMPARISONS. 231 

ditions, to millstuff nearly free from admixture with minerals 
prejudicial to a very complete amalgamation of the gold 
which it contains. /j 

The retort percentage is largely dependent upon the hard- l\((^ 
ness of the millman's hands. Apart from this factor, it be- - . ' '" 
comes proportioned to the coarseness of the gold particles and 
the thoroughness of their amalgamation — that is, the com- 
pleteness of the alloying of the two metals, for an amalgam 
is an alloy formed by mercury with another metal in the cold. 
• The fineness of the bullion is an excellent index of the 
docility of the ores, because those which carry their gold with- 
in an incasement of pyrite yield retorts containing impurities, 
of which silver is the most frequent and least undesirable. 
It certainly does so happen that if you go through the list of 
these milling centers, taking them in the order of their 
bullion fineness, beginning with Clunes (975 per 1000) and 
ending with the Thames (640 per 1000), you will have the 
exact scale of their gradation from the most distinctly free- 
milling, docile ore to the most decidedly difficult and refrac- 
tory. Furthermore, speaking generally, I may mention, as a 
striking coincidence merely, that this figure will be roughly 
indicative of the percentage of extraction, because this last 
is a corollary from the fact just noted. Thus a Gilpin County 
mill will extract just about 78 per cent., those of the Thames 
rarely reach 64 per cent., those of Grass Valley usually exceed 
84 per cent. As a rough approximation of a probable extrac- 
tion, the fineness of the retort gold affords in this way an 
interesting guide. 

The variation in mercury consumption is the widest of all, ^j 
from 5 grains to a pound per ton. The minimum figure at ^^-l^^ 
Clunes is traceable to the fact that no mercury is used in the '^ ^^ 

mortar and blanket tables replace the copper plates ordinarily ' * 
employed on the outside. Mercury is only used in wells and 
in the barrel which treats the blanketings. Rich ores entail a 
greater consumption than poor because more mercury is used. 
Charters Towers and the Thames give the maximum loss 
because of the addition of mercury in the pans, where it 
becomes floured and sickened* by being present while base 
sulphides are undergoing grinding. 

♦See Chapter XV. 



232 STAMP MILLING OF GOLD ORES. 

Water consumption is primarily dependent upon the rate 
of crushing. It is increased by the addition of processes 
supplementary to the ordinary battery treatment. Blankets 
require more water than plates, hence the high figure at 
Clunes. Slow crushing needs a small supply, therefore the 
low consumption in Gilpin. 



The Elmore Mill, Rocky Bar, Idabo. 
Finally we reacli tiiat most important factor of all, the 
character of the ore. It is a cynical truism to say that the 
milling process should be adapted to the nature of the ore, 
because, unfortunately, it reminds one at once of the scores of 
expensive plants which are rotting upon the hillsides of our 



COMPABISONS. 233 

mining regions as the consequence of a perverse disregard of 
this obvious relation. Ordinarily it is best to have a mine 
possessing certain ore reserves before starting to erect a mill. 
Moreover, it is safer to have representative lots of ore tested 
at a neighboring reduction plant than to design a line of 
treatment on the insecure basis of a few laboratory experi- 
ments. The old folly still survives. Men of intelligence try 
a few grains of ore in a test-tube and then hasten to telegraph 
the order for a $50,000 mill. English directors are particu- 
larly prone to placing blank orders for "a gold mill" in the 
hands of fashionable machinery firms. Many of the resultant 
failures are put upon the broad back of an old well-tried pro- 
cess, which is further loaded with the incubus of incompetent 
management and incomplete equipment. The average mine- 
owner not infrequently prefers fooling with '* a man • and a 
process " to spending a thousand dollars in properly directed 
experiments. Consequently there are many who, complain- 
ing of the unsatisfactory extraction in the batteries of a 
badly arranged or incompletely equipped stamp mill, rush 
headlong into the maze of complicated, half -understood leach- 
ing processes. Their last end is often worse than their first. 

In the above list of gold-mining centers there are at least 
two, both colonial, instances of the application of stamp 
milling to ores essentially unsuited to such a process of 
reduction. Some one put up a stamp mill long ago and the 
others have copied the first builder, because probably the ores 
that went to that first mill were so rich that in spite of an 
unsatisfactory percentage of extraction the mine paid divi- 
dends. After that no one wanted to be guilty of unprejudiced 
investigations resulting in a new departure. In other in- 
stances, as in Gilpin County, while the ores of most of the 
mines and most of the ores of any particular mine are 
susceptible of successful treatment by the process in vogue, 
there is milch millstuff undergoing amalgamation and supple- 
mentary concentration which should be subjected to well- 
devised concentration accompanied by incidental amalgama- 
tion. This is being realized of late, so that the stamp mill, 
the concentrator, and the smelter in the valley will each 
receive that proportion of the ore production which is clearly 
designed, from a metallurgical standpoint, to go to them. 



234 STAMP MILLING OF GOLD ORES. 

Incidentally mention may be made of the use of the stamp 
mill for the recriishing of the tailing:s from jigs. It has been 
generally supposed that jig sands are not suited to battery 
treatment. Their regrindinfe has been given over to rolls, 
Huntingdon mills, and a multitude of patent pulverizers. 
The successful work accomplished at several mills in the 
southwestern part of Colorado has shown that both the Hunt- 
ingdon mill and the stamps are well adapted for this purpose ; 
that the former is, because of its construction, suited for erec- 
tion in mountainous regions ; and that the stamp mill will 
treat jig sands satisfactorily when due regard is paid to regular 
feeding. Under such conditions it has a capacity and gives 
results rendering it suitable for such tailings coming from the 
jigs as consist (5f quartz sands containing free gold and have 
been previously deprived of those brittle sulphides which 
make slime. The crushing of ore preparatory to jigging is, 
on the contrary, a work which it is rarely advisable to give 
to the stamps, because, being badly adapted to sizing the pulp, 
they deliver it in a poor condition for concentration. 

Certain districts in both hemispheres make only a lame 
effort to arrest the gold-bearing sulphides escaping in the 
tailings of the stamp mill. The ordinary practice is to employ 
two vanners per battery, because it is the ratio which has 
been found adequate to the treatment of the ores of the 
Californian mines, where the vanner first came into success- 
ful use. Guided by such a venerable rule of thumb, the mill- 
owner equips his place in a stereotyped fashion, regardless 
of the fact that his ore may carry a several times larger 
percentage of sulphides. As a rule, the number of vanners is 
insufficient because it is not recognized that they are machines 
which to do close work cannot be crowded. Nor, on the 
other hand, should it be expected that a delicate concentrator 
such as this can give satisfactory results when called upon to 
treat pulp varying in size from 30 mesh to impalpable slime. 
The importance of sizing, itself an inexpensive operation, is 
almost universally overlooked in English-speaking countries, 
to the heavy detriment of the work done by vanners and 
shaking tables. 

The successful introduction of certain leaching processes, 
whether for the reduction of concentrates or the treatment of 



COMPARISOKS. 235 

tailings, has caused many millmen to lose sight of the desira- 
bility of extracting the values in the ore as soon as possible 
and not expecting to redress the unsatisfactory results of a 
poor battery amalgamation by a subsequent supplementary 
process. The pride of many millmen over the obtaining of 
gold from tailings which have been permitted to go through 
the mill without yielding up a proper percentage of their con- 
tents, and the satisfaction expressed over the work afterward 
done by the cyanide tanks or the chlorination vats, is due to a 
disregard of one of the first postulates of good milling, namely, 
to catch the gold as soon as possible. Do not send specimen 
ore to the battery if it can be treated in a hand mortar ; do 
not let the gold get into the outside plates if it ©an be arrested 
within the battery itself ; get it on the amalgamating tables 
rather than on your blankets or in your wells ; do not depend 
on your concentrators if you can save the values by amalga- 
mation, nor neglect the care of the vanners because the tail- 
ings are to undergo further treatment. A sportsman is not 
considered a good shot because he misses the bird with his 
first barrel and brings it down with the second. ( 

In the foregoing glance over the field of activity covered by 
the simple mechanism of the stamp mill, the Witwatersrand 
has not been included because the practice of that region is 
yet in its formative stage and is destined to undergo impor- 
tant changes of development. At present it offers a magnifi- 
cent opportunity for the utilization of the best experience of 
the older districts of the world. The application of the stamp 
mill to the conglomerates of the Rand serves further to 
accentuate its wide range of usefulness. It is this adapta- 
bility to ores of great diversity which enables it to hold its 
own in the face of the newer devices constantly offered by 
the restless inventive genius of the age. Between a simple 
white quartz carrying loosely attached particles of coarse, 
•clean gold and a compact pyrite wherein the gold is invisibly 
enveloped, there is a divergence so great as to emphasize the 
elasticity of a process which, despite the encroachments of 
the smelter on the one hand and wet methods on the other, 
remains the simplest and cheapest way of extracting gold as 
yet devised by the ingenuity of man. 



( 



CHAPTER XVII. 

Mills and Millmbn. 

In looking back over the visits to so many mills in localities 
so essentially unlike it is impossible not to realize that the 
variations in milling; are as nothing compared to the varia- 
tions in mankind. The gathering of data by means of inquiry 
and observation is an undertaking the pleasure of which is too 
often swamped by the labor consequent upon the drawing out 
of information from persons unwilling or unable to give it. 
Much as technical science has made itself at home in every 
Anglo-Saxon community, there yet remain many who cannot 
comprehend the gathering of knowledge for its own sake, and 
only too often the inquirer is received with a suspicion which 
is as annoying as it is detrimental to the objects of his pursuit. 
In gathering data at the various milling plants I have found, 
as a rule, that the establishments which were the most 
systematically conducted were the readiest to grant the in- 
formation required ; that the most refractory were the most 
ignorant of what they were really doing, and that in the latter 
case frequently the questions asked were such as the mill 
manager himself had never put to himself, and to which there- 
fore he could not reply without realizing, and for the first 
time, what he actually was doing. 

The differences between the abilities of millmen and the 
management of mills would be absurd were it not depressing. 
It is depressing because there seems to me to be no excuse for 
consigning the handling of an ore-reduction establishment to 
the hands of a carpenter or a miner ; it is depressing because 
it appears such a blunder to exercise all skill and judgment 
in the mining of the ore and then to overlook the importance 
of the subsequent proper extraction of its valuable contents* 

(236) 



MILLS AND MILLMEN. 237 

That such things should be is due to the fact that the stamp 
mill is usually considered a very simple mechanism, capable 
of regulation by almost anyone. The apparent simplicity of 
the treatment has prevented technical men from making it 
the subject of study which it appears to the writer to fully 
deserve and invite. And this the more since, in spite of the 
encroachment of other more intricate processes, the stamp 
mill seems to be destined to a career of further long-continued 
usefulness. 

It is, indeed, true that fire-reduction processes are encroach- 
ing on wet methods, more particularly in the case of complex 
ores ; it is also true per contra that chlorination, cyanidation, 
and other leaching processes, are actively competing for the 
mills tuff which might otherwise go to the battery, yet it will 
generally be found that the diversion to other reduction works 
of ores suitable to stamp milling is comparatively slight and 
confined to such as are essentially unadaped for amalgamation. 

The simplicity of the machinery of stamp milling, its long- 
proven adaptability to various docile ores, and the noteworthy 
cheapness of the treatment, render it preeminent among 
metallurgical methods of gold extraction. We hear con- 
tinually of new inventions for the rapid pulverization of ore 
and the easy extraction of its contained gold. New processes 
are born daily but their mortality is very depressing. It may 
be that some heaven-sent revelation may some day enable the 
metallurgist to extract 100 per cent, of the value in an ore at 
a cost insignificant when compared to the magnitude of the 
result, but we may well have grave doubts. The story of 
metallurgical progress proves it to have been evolutionary 
and not revolutionary. It is by the gradual improvement of 
established methods rather than by the sudden application of 
new and untried processes that success is soonest attained 
and longest maintained. The record of the chlorination 
process in America affords an instructive illustration. Al- 
though employed at Grass Valley,, Oal., as early as 1857, it has 
taken nearly 40 years to bring the knowledge of the best 
methods of its application to such a stage as to render its use 
technically successful and economically safe. From the slow 
and simple practice first borrowed fromPlattner it has passed 
through many changes until to-day in the improved barrel 



288 STAMP MILLING OF GOLD ORES. 

process it has taken a firm hold upon a certain part of the 

metallurgical field. Cyanidation, on the other hand, illustrates 

in a marked manner the earlier struggles of all leaching 

methods, and its checkered career is one commonly shared by 

delicate chemical processes of ore reduction in course of sue- 1 

cessful evolution from uncertain beginnings. 

The more familiar methods of stamp milling have long 
ago grown out of the uncertainties of early development. 
How gradual and how great that development has been, can 
only be realized by tracing the workings of human ingenuity 
through many centuries of endeavor from the first conception 
of the simple savage with his stone implement to the com- 
pleted mechanism of the modern millwright. It is a story of 
the survival of that which, was fittest for the work of extract- 
ing that gold which has ever been the ignis fatuus of human- 
kind ; it is a tale of the adaptation of methods to different 
environments, which have included within their limits the 
arid deserts of Western Australia and the snow-clad summits 
of Colorado. The inquiry into the divergent practice of 
scattered regions has indicated clearly that the key to the 
apparent contradiction involved in methods which seemed so 
opposed in principle, is to be found in an earnest attempt ta 
adapt methods to local conditions. The real principles — 
the bedrock ideas — of stamp milling are unchanging, but 
their successful application has created a growth of local 
variations which are discordant only when viewed without 
due consideration to diverse conditions. 

No method of ore reduction is perfect ; the metallurgist can- 
not dare to consider any particular process the ultima Thule of 
technical progress, and in spite of the long period of develop- 
ment through which the stamp mill has passed it needs no 
hesitation to assert that no milling plant exists which is 
perfect. It is hardly necessary to add that in the course of 
inspecting more than a hundred mills the writer has fre- 
quently met with those who considered their own plant 
impossible of improvement. The w^orst of all ignorance is the 
ignorance of our ignorance. The most experienced millman 
is usually very well aware that the improvements made in 
the past are merely the measure of the betterment attainable 
m the future. 



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MILLS AND MILLMBN. 239 

Engineering is applied common sense ; and we do not 
wonder to find that many of the alterations which render the 
mechanism of to-day superior to the rude appliance of 
yesterday, are striking more for their ingenious utilization of 
simple principles than in their embodiment of complex con- U^ 
siderations. If I should pick out any part of the stamp mill / ' 
as more particularly illustrative of this truism, I would select 
the amalgamating table. Everywhere the sloping surface of 
copper plate or the outspread stretch of blanket serves as a 
gold-saving appliance to supplement the variously successful 
extraction in the mortar box. The amalgamating tables are 
generally arranged in two series, of which the upper one, v/^ 
called the apron, has a width approximately equal to that of /^'^ 

the issue of the battery, and varies only between narrow 
limits, from 4 feet 4 inches to 6 feet. Its length shows greater 
extremes, reaching from 2 or 3 feet to as much as 16 feet. 
Then comes the lower table, also termed the sluice or tail- 
plate, which rarely is of a size equal to the apron, but more 
ordinarily, especially in California, has a width of about 20 
inches, and a length of from 10 to 20 feet. 

The arrangement of these obviously very important parts 
of the gold-saving appliances of a stamp mill usually invites 
condemnatory criticism; sometimes even cynical derision. 
For what, I would ask, can be more absurd than to pass the 
pulp over a surface 5 feet wide, extracting some of the coarse 
gold, and then to compel it to run within the confined width 
of 20 inches in the expectation of arresting gold less coarse, 
and, therefore, more diflScult to arrest, than that which has 
been caught above ? It ought to be obvious to ordinary com- 
mon sense that the gold to be saved below the apron is of a 
kind which, because it has escaped the first effort to catch it, 
is necessarily more elusive than that previously saved, and 
that therefore conditions more favorable to its arrest than 
those obtained on the apron-plate are required. The common 
actual practice would point to the contrary view, since, after 
the pulp has slowly washed in successive waves over the wide 
surface of the upper table, it is rushed through the narrowed 
confines of the lower. In several instances, moreover, I have 
actually found the sluice-plate to slope at a steeper angle than 



240 STAMP MILLING OP GOLD ORES. 

the apron, with the result that the edges of the copper surface 
were scoured and their amalgam rubbed off by the rapid flow 
of sand and water. 

What is evidently needed is a reversal of the ordinary 
method ; the spreading of the pulp over an area of increasing 
width rather than its confinement within a narrowing space, 
such as makes the ordinary sluice much more of a launder 
for the rapid conveyance of the pulp than a device for the 
saving of any gold which may pass over it. 

When traced back through the years it will be found that 
the use of this quite absurd arrangement sprang from the 
fact that it was borrowed from the placer miner, and that it 
originally formed a part of the alluvial diggers' gold-saving 
apparatus. River mining preceded vein mining, and in trans- 
ferring his energies from the one to the other the miner was 
inclined to use as much as possible of the plant needed in the 
one industry for employment in the other. The sluice-plate in 
the mill is a modified form of the sluice-box of the gulch. 

Notwithstanding the marked improvement made in the 
mechanical appliances of the typical western American stamp 
mill, tending chiefly to greater automatism of treatment, this 
particular feature remains a blemish and a defect for which 
there is no excuse save that " everlasting monkey" in man, 
in virtue of which the millman copies the methods of his 
neighbor without proper investigation into its merits or in- 
tention. No part of the apparatus of the mill is so cheap as 
the amalgamating plate. An old plate is always worth more 
than a new one, because of the gold which it absorbs in 
process of use. The fact that the bottom of a plate catches 
any amalgam, in amount however small, should be sufficient 
evidence that some is escaping which could be arrested by a 
further elongation or enlargement of the amalgamated surface. 
I have never seen a mill which had too much plate area ; I 
have seen several which had too little. And in this connec- 
tion I may be pardoned for mentioning the fact that in two 
instances where I have been called upon to endeavor to 
suggest means for an improvement in the gold extraction of 
a stamp mill, I have succeeded to a notable degree in doing 
so by the simple rearrangement or extension of the amalga- 
mated copper plates. 



MILI^ AND MILLMEN. 241 

No completeness of treatment is likely where no accurate 
knowledge of the results is secured. In most stamp mills 
accurate and systematic sampling is unknown. The estima- 
tion of the percentage of extraction is usually mere guesswork 
based on an occasional assay of a sample carelessly taken. 
As I write I remember only three instances of proper care in 
this department. They were the Harrietville mill (Australia) 
and the Empire and North Star mills (California). Establish- 
ments of such a magnitude as the Homestake in South 
Dakota merit condemnation for an absence of proper system 
in this regard. Too many millmen are in utter ignorance of 
the real character of the results which they are obtaining, an 
ignorance which is as consolatory to a complacent self-satis- 
faction as it is injurious to the real interests of the proprietors 
of the mill. 

It is impossible to become familiar with the work being 
done at stamp mills scattered the wide world over without 
being impressed with the frequent waste of time and money 
incurred in the carrying out of experiments at one locality 
which have been made long previous in some other district. 
You may, for instance, find the mills of one region introducing 
for the first time a form of tappet which was thoroughly 
tested and found altogether undesirable at some other place a 
few years previous ; or a millman in Australia may be design- 
ing a mortar to do certain work and of a pattern practically 
identical with one which was tried and found unsuitable in 
California a decade ago. The waste of experience, the going 
over ground a second time which has once been proven, the 
failure to use the results of careful tests made by others, this 
presents a subject which must often have impressed and 
saddened those who have investigated the struggles and en- 
deavors which have been the stepping-stones to metallurgical 
development. 

There is a curious lack of intercommunication between 
those who direct the operation of mines and mills in various 
parts of the world. Although this is overcome in some 
instances by the reading of The Engineering and Mining Jour- 
nal^ the Transactions of the American Institute of Mining 
Engineers^ etc., it nevertheless forms a serious bar to intelli- 
gent advancement. At many of the largest milling establish- 



242 STAMP MILLING OF GOLD ORES. 

ments you may find men in charge who are even proud of 
never having stirred from their own locality for a period 
which covers 10 or 20 years, and may sometimes reach a life- 
time. The world is moving; technical progress has made 
strides which have stamped our modern civilization with 
characteristics unknown to previous history. Is it not lament- 
able that men should stick in the mud in their own little 
hamlet, careless of what others may be doing, heedless of op- 
portunities for improvement, because their minds are dulled 
by a sleepy belief that the workings of their own little 
practice is incapable of betterment ? 

Rhetoric apart, and in all seriousness, would it not prove 
profitable to many large proprietaries, on both sides of the 
equator, if they would occasionally send their captains of 
industry to investigate the methods of other regions, to see 
and to study and possibly imitate, the results of experiments 
and improvements made by others in their own line of work? 

There is reason to deplore the general rarity of careful 
investigation. The choice of screens, the use of certain 
chemicals, the employment of different metals for shoes and 
dies, matters such as these are ordinarily determined at 
haphazard, or by the ancient and venerable rule of thumb. 
Occasionally tests are made, but of such the majority are 
hopelessly vitiated by want of system and lack of accuracy. 
A man, for example, tries to find out which kind of screen to 
employ, so he fits up one battery with the one and the next 
battery with the other. He forgets that owing to the dis- 
position of the rock-breaker one battery gets more of the 
fines than the other ; he omits to note that in one mortar the 
dies are worn down and in the other they have been lately 
replaced, so that the issue in the two batteries differs by a 
couple of inches. A test under such circumstances is a snare 
and a delusion. The ordinary result is a mass of contradictory 
data which are only helplessly confusing. 

There is a common repugnance among millmen to do any 
experimenting. This springs largely from the known futility 
of previous tests which were inconclusive because carried out 
under unequal conditions. For such a feeling those are to 
blame who, imperfectly understanding what they were about, 
directed work whose object their subordinates failed to 



MILLS AND MILLMBN. 243 

• 

appreciate. The average millman may be stubbornly opposed 
to the trying of new methods which are only partially com- 
prehended by those advocating them, but he is, I believe, 
fair-minded enough to be willing to carry out tests which are 
based on an adequate comprehension of the conditions of the 
case. 

On the other hand, careful tests made to determine a 
certain factor in the milling problem, when they are made 
under equal conditions, such as guarantee reliable results, 
are invaluable. In every mill there should be a reason for 
doing anything in a particular way. The millman should be 
able to show cause for having the height of drop just what it 
is ; he should be in a position to explain why he cannot re- 
commend a depth of discharge a couple of inches more or less 
than that actually in use, and so on. If uncertain why 
particular methods are in vogue he should by accurate experi- 
ment, as soon as circumstances permit, confirm their correct- 
ness, or replace them by others which he finds better. The 
practice of guessing twice and then dividing by two for 
further accuracy is one which can only conduct a man into a 
hopeless bewilderment between what he do€fs know and what 
he thinks he knows. 



CHAPTER XVIII. 

The Future of the Stamp Mill. 

The stamp mill has suffered much in repute from the 
frequent failure to adapt the design of the mill to the capacity 
of the mine or the character of the ore. Some men order a 
reduction plant like others order a dinner. They go to the 
representative of a well-known machinery firm and tell him 
about the mine and the nature of the ore, and then leave the 
choice of the arrangement of the mill to him just as a man 
might enter a first-class restaurant and tell the head waiter 
to serve a good dinner, leaving the menu to his judgment. 
Mine-owners often hate to expend a thousand dollars in 
advice or experiment previous to the erection of an ore- 
reduction establishment, but really enjoy ordering a hundred- 
thousand-dollar mill, which may no sooner be completed and 
at work before they find that the process is unsuitable or the 
ore supply inadequate. Hence the frequent monuments to 
folly which dot our western hillsides. Hinc illce lacrimce when 
shareholders inveigh against processes which prove a delusion 
and mills that fail to yield dividends. The causes underlying 
the miscarriage of milling plants are not obscure. If they 
are hidden from the unwise and imprudent, they are daily 
revealed to mere babes in metallurgical experience. 

Only recently a typical instance came across my way. A 
mine-owner, who is ordinarily a real estate broker, went to 
the manager of a machinery manufacturing concern and, 
exhibiting a piece of ore, told him that he wanted a mill to 
treat material of which that is a sample. The ore carried a 
large percentage of pyrites, but the gold associated with it, 
so said the real estate broker, was entirely amalgamable. 
The machinery man advised him to put up a long-drop, slow- 

(244) 



THE FUTURE OP THE STAMP MILL. 245 

speed stamp mill, supplemented by concentrating tables. It 
was so ordered. The mill was shipped in due time, and was 
erected in the wilds of Idaho. From the very start everything 
went wrong. The mill did bad work, and the mine-owner 
anathematized the machinery fellow ; the latter excommuni- 
cated the former. Not long afterward I happened to be on 
the ground and found the facts to be simple. When the mill 
was ready to start it was fed, not with the hard pyritic quartz 
such as the original sample shown, but with very soft surface 
gossan. The feeding was low. The stamps with their long, 
slow drop came crashing through the thin cover of soft 
material. Cams began to break, shoes went into splinters, 
and screens were destroyed in a day. The mill was over- 
hauled, the drop was shortened, and the cams replaced by 
others. The arrangement of the mill was gradually so modi- 
fied that it became a hybrid between a short-drop, quick-speed 
form of battery and its original design. Better results were 
obtained. Then a new superintendent came upon the scene. 
Work at the mine was transferred from the surface open cuts 
to deeper levels. Hard pyritic ore was sent to the mill. The 
crushing capacity of the stamps was diminished, and the 
amalgamating tables, their slope remaining unchanged, were 
unable to clear themselves. Extraction declined out of sight, 
fhe machinery firm was again pilloried. About this time 
the undertaking got into financial trouble and the plant was 
hired by a neighboring company, which was able to treat its 
(similar) surface ores in this mill with marked success. It 
all sounds foolish enough, but pity 'tis 'tis true, and not once 
only, but many times. 

The machinery man, however, often deserves censure also. 
There are establishments which have what they call a '* stand- 
ard" type of mill which they highly recommend for the 
reduction of ore running through a whole gamut of differing 
composition. Like the iron bed of Procrustes, to which the 
wayfarer had to suit his length at the risk of summary abbre- 
viation or painful elongation, so the manufacturer expects 
the ore to adapt itself to his mill or choose between being 
labeled unprofitable or refractory. These are diflBculties 
which could be readily overcome. The failure of a plant 
hurts the reputation of the firm that supplied it no less than 



246 STAMP MILLING OF GOLD ORES. 

it decreases the value of the mine. It would be a profitable 
thing for both parties in the transaction if, it being agreed 
that the order will be placed, they could agree upon an 
investigation of the ore by a competent authority with a view 
to determining the best treatment, the expense of such 
investigation to be divided between them. 

More than once, in the course of the investigations upon 
which these studies of milling have been based, there has come 
the question. Is the stamp mill likely to survive amid the inven- 
tions daily heralded from the Patent Office ? Will it continue 
to compete successfully with the multitudinous pulverizers 
and amalgamators, together with the unending array of new 
processes which the restless brain of man brings forth from 
day to day ? To the writer, looking over the field of metal- 
lurgical competition and cognizant of the fearful slaughter 
that befalls the army of ill-conceived and half-completed 
machines which their inventors fondly imagine competent 
to revolutionize ore reduction, there comes a strong belief 
that the stamp mill is destined to survive amid much com- 
petition and to enjoy a career of further long-continued 
usefulness. Often enough some other process or some differ- 
ent pulverizing mechanism is claimed to do better work than 
the stamp mill. Occasionally this is true for particular ores 
under particular conditions, but just as frequently it is due 
to the fact that in making the test the stamp mill has been of 
unsuitable design or has been unintelligently handled, so that 
the comparison has been unfairly made. There is, believe 
me, just as much difference between a model stamp mill 
properly directed and an imperfect one badly managed as 
there is between the latter and some one or other of the 
newer processes of ore reduction. I have known a leaching 
process put in rivalry with an imperfectly equipped or im- 
properly managed stamp mill, and the former has given a 
percentage of extraction greater than the latter, but in the 
sequel it has become evident that the stamp mill, when it has 
been subsequently provided with the needed appliances and 
superintended by the necessary man, has surpassed the leach- 
ing process as much as the last previously surpassed the 
stamp mill. 



THE FUTURE OF THE STAMP MILL. 247 

One feature of the stamp mill stands out cleariy when 
instituting a comparison between it and other pulverizers, 
namely, it is a crushing and an amalgamating, a reducing and 
an extracting machine combined. This distinctive feature 
has enabled it to hold its own against other newer inventions 
for pulverizing ore and to meet the fierce competition of so 
many more complete and more complicated amalgamating 
machines. In the two extreme types of the stamp mill, so 
often referred to, we have seen, on the one hand,* how an in- 
creased degree of amalgamation has been secured at a sacrifice 
to rapidity of pulverization, and, on the other hand,f how 
fast crushing has been aimed at and battery amalgamation 
made subservient to the desire for the expeditious treatment 
of large quantities of low-grade millstufF. In the one case 
the mortar has been enabled to do work otherwise beyond its 
scope ; in the other, ore has been handled with commercial 
success which otherwise could not be profitably reduced. It 
is interesting to note, however, that even in California and 
South Dakota, where the stamp mill is so designed as to be 
essentially a rapid pulverizer, the amount of gold arrested 
inside the mortar forms about one-half of the total extraction. 

This feature of the stamp mill is one overlooked by many 
who daily direct their inventive genius to the discovery of a 
mechanism which shall surpass the clumsy contrivance whose 
reverberations echo from Coolgardie to Colorado. The steam 
stamp, for instance, eminently successful as it has shown 
itself in the quick reduction of the copper ores of the Lake 
Superior region, has not proved satisfactory in its application 
to gold ores. Why ? Its crushing capacity per horse-power 
consumed is much ahead of the ordinary stamps. True, but 
it does not permit of amalgamation going hand in hand with 
pulverization, the force and rapidity of the discharge are un- 
favorable to fine crushing, the extreme violence of the agita- 
tion inside the mortar prevents the introduction of amalgamat- 
ing plates, and, as a whole, it notably fails in giving the 
conditions required for successful milling. 

The same question crops up in the discussion of the use of 
heavy stamps. The Alaska Treadwell Company made numer- 

* In Oilpin County, Colorado, 
fin South Dakota and CaUfornia. 



248 STAMP MILLING OF GOLB OBES. 

0118 experiments, and found 1000 pounds the practicable 
limit. Heavier stamps might crush faster, and indeed did so, 
but this very fact resulted in the rushing of the ore through 
the battery so rapidly that opportunities for that contact 
between the gold and the mercury which is the essential 
requirement for amalgamation, were lessened to such an 
extent as to seriously diminish the percentage of extraction. 
The mill became a good pulverizer, but a bad amalgamator. 

Nine-tenths of the patent pulverizers and new amalga- 
mators thrust before the public through the medium of bom- 
bastic advertisements are crippled by a similar defect. Where 
rapid pulverization is secured an ineffectual effort is often 
made to secure concomitant amalgamation, but in most cases, 
the cutting up of the mercury introduced into the machine 
causes so much "flouring" as to render a heavy loss of both 
mercury and gold unavoidable. I. have before me, as I write, 
a typical description of a machine of this kind. The author 
of the description, who possesses merely a bowing acquaint- 
ance with his subject, emphasizes the statement that it is aa 
" evolutionary machine " which for the first time utilizes a 
new principle, namely, the "atomic pulverization" of the 
quartz and the complete liberation of the gold. I happen to 
know that that " evolutionary machine " lies resting in many 
a mill where it can now be purchased on the basis of scrap 
iron. Let me mention another example. Lately, while going 
up one of our picturesque Colorado cafions, I visited a plant 
which has been rearranged. The man in charge informed 
me, with unnecessary emphasis, that mercury was a " robber 
of gold," and that his (the speaker's) "new system," which 
was to utilize *' hot water and air," would plainly demonstrate 
such to be the fact. I enjoyed the subsequent conversation. 
That man was as deliciously ignorant of what the stamp mill 
can do and how it does it as the dog that bays at the moon is 
of astronomy. He had persuaded a few stockbrokers to 
introduce his "new system," of which what was useful was as 
old as the hills, and what was essentially absurd and imprac- 
ticable was his, entirely his. Such instances are not un- 
common. They happen weekly in spite of frequent doses of 
bitter experience. They explain why so many mills are* 



THE FUTUBE OF THE STAMP MILL. 24& 

rotting in the sun and rusting in the rain— object lessons 
whose teaching is as unheeded as the whistling of the wind 
through the neighboring pines. 

Not that one would suggest that mechanical ingenuity and 
metallurgical experience will fail to better our present 
methods. No ; but that betterment will be brought about by 
men who are cognizant of what is being done already and of 
how the present practice was evolved rather than by those 
who are contemptuous of a process whose principles and ap- 
plication they have scarcely tried to comprehend. 

Therefore, in conclusion, to millmen and metallurgists, 
fellow-students in a field of endless interest, I would say: 
Let us endeavor to use the stamp mill intelligently, to under- 
stand the why and wherefore of every one of its successive 
operations, and to lose no opportunity of applying any con- 
trivance or modification which experience sanctions and ex- 
periment corroborates. That done, we shall have done our 
little best as best we can. In the meantime the inventive 
genius of this great mechanical age ruminates apart in an 
earnest. effort destined in due time to evolve something better 
wherewith to catch the yellow gold whose want is the pain of 
some, whose excess is the curse of others. 



t 



A GLOSSARY OF STAMP-MILLING TERMS. 



In the following explanation of the terms which have been used 
in these articles the first word will be American and the second the 
English or Australian equivalent. The American terms are often 
used outside of the United States. 

Stem or shank, — The vertical rod or shaft of wrought iron which 
carries the stamp at its lower end. It is the handle of the hammer. 

B088 or tophead, — ^A heavy cylindrical piece of iron (usually cast 
or steel) into the top of which the stem fits and into the bottom of 
which the shoe is inserted. It is the body of the hammer into 
which the handle fits and which also gives heft to the blow. 

Shoe or head, — The wearing part of the stamp which comes in 
contact with the ore and does the actual crushing. The hammer- 
head. 

Die or false bottom, — The iron, a flat hexagonal or cylindrical 
piece, upon which the ore is crushed. It is the anvil. At Clunes, 
in Victoria, it is called the " stamper bed." 

Cam or wiper, — The curved lever which lifts the stamp and is 
therefore sometimes called the " lifter." 

Tappet or Disc, — The collar under which the cam is inserted so as 
to lift the stamp. 

Mortar or coffer, — The box, sometimes of wood but lined with iron, 
usually solid cast iron, inside which the operation of crushing takes 
place. It is the mortar of which the stamp is the pestle. Also 
called ** stamper box." 

Screen or grating. — The perforated metallic plate or wire cloth 
through whose openings the crushed ore is discharged, and which 
is intended to prevent its exit until crushed to a degree of fineness 
supposed to be regulated by the number and size of the openings in 
the screen. 

Ties and tyea, — The latter to be preferred. A Cornish term for 
the sluice-boxes used for the extraction of the heavy sands in mill 
tailings. 

Tailings, — The waste from the mill ; the pulp after the gold has 
been extracted by mill treatment. In Gilpin County, Colorado, it 
is applied, erroneously, to the concentrates saved on shaking tables. 

Chuck-block and chock-block, — The first to be preferred. The 

(250) 



A GLOSSAKY Or STAMP-MILL PRACTICE. 251 

wooden block or board which is attached to the bottom of the screen 
so as to raise the depth of the issue and act as a false lip to the 
mortar. 

Discharge or issue. — The expulsion of the pulp from the mortar. 
It is also used to designate the distance from the bottom of the screen 
to the top of the die, because this flt?ure determines, more than any 
other factor, the rapidity of the expulsion of the pulp. 

Battery.— Any single mortar with its stamps (usually five) 
complete. The words ** stamper" and '* head" are also employed in 
the colonies in place of ** stamp." 

Mill or machine. — The latter is largely confined to Queensland, 
although ^'crushing machine" and ^'battery" are used synonymously 
with *' mill" in other parts of Australia to designate the reduction 
plant as a whole. 

Traps or wells. — The troughs and catch-pits, whether carrying 
mercury or not, which are used to arrest escaping amalgam, etc. 
The word " trap" should be confined to the deep boxes unprovided 
with mercury, and the word "well" to the transverse troughs which 
do contain it. At Clunes the word *' boxes" is used, while elsewhere 
in Australia '* ripples" is a term given to shallow wells as dis- 
tinguished from the deep ones. 

Pan or dish. — ^A copper or galvanized iron utensil used for washing 
gold ore and gravel so as to separate the heavy gold by a shaking 
motion. It corresponds to the Cornishman's vanning shovel. 

The Australian uses the word ^' crushing" where the American 
says ''mill-run" to designate the treatment of a given quantity of 
ore. The Colonial employs "mullock tip" and ''spoil heap" as 
synonymous with the American " dump" to describe the accumula- 
tion of waste rock coming out of the mine. 

The American prefers "quicksilver" where the Australian uses 
"mercury." In the Colonies a "flask" of mercury contains 76 
pounds avoirdupois and in the United States, a " bottle" or " tank" 
of quicksilver contains 76)^ pounds. The Colonial calls the ore sent 
to the mill " stone," the Englishman " millstuff " and the Colorado- 
Cornishman "mill dirt." 

The growth of travel and the rapidity of modern methods of com- 
munication tend to bring about a uniformity in technical terms 
which is convenient and desirable. 



INDEX. 



\l^rt ' I I - >'■ -' PAGE. 

Acid water, action on plates, 182 

screens 24, 173, 176 

lime as a corrective 124 

Alaska Treadwell mill 225 

Amador County, Cal 57 to 74 

Amalgamating barrel 1 10 

Amalgamating tables ; see also plates. 

arrangement of ... 40, 64, 71, 91, 141, 162, 164 . >-->;? 

drawings of 43, 63 

plain copper .... 17, 91, 101, 123, 139, 158, 19a 

silver plated 40, 54, 63, 191 

slope of 17, 18, 40, 64, 91, 136, 139, 164 

variety of metals used 176 

width of 40, 43, 56, 71, 91, 193, >^^ 

Amalgamation, distribution in mills . 44, 70, 92, 104, 111, 146, 159, 189 

effect of temperature 126 

inside the mortar H, 40, 62, 69, 81, 158, 190 

Assaying, importance of 163 

Australian practice reviewed 196 

Ballarat district, Victoria 119 to 132 

Barrel, drawing of amalgamating barrel 110 

Barrel treatment of blanketings Ill, 187 

Bendigo district, Victoria 133 to 153 

Black Hills, So. Dakota 75 to 101 

Blanketings, treatment of 65, 111, 141, 171, 187, 192 

Blankets and blanket tables. 

Australian practice 109, 136, 141, 165 

Calif ornian practice 65 

Colorado practice 18 

compared to amalgamation 191, 194 

New Zealand practice 171, 177, 186, 192 

origin of 190 

treatment of washings 65, 111, 141, 171, 187, 192 

width of .18,109,116,19a 

(252) 






INDEX. 253 

Brittania United mill, Victoria 121, 125 

Buddies 104, 109 

Bumping table 18, 30, 90 

Burdekin mill. Charters Towers, Queensland 203 

Caledonia mine, Black Hills, So. Dak 76 

Calif ornian milling practice, compared to Colorado 2, 5, 69 

described 57 to 74, 35 to 56 

table of mills 36, 58 

Cambria mill, Thames District, New Zealand 169, 182 

Catherine mill, Bendigo, Victoria 135, 137 

Charters Towers district, Queensland 198, 202 

Chlorination 162, 203 

Chock-block 62,69, 81,^*' 

Chrome steel for castings 39, 66, 93, 208 

Clay in ores 169 

Clean-up, Colorado method 20 

Californian 45, 49, 64 

Australian 109, 143, 158 

New Zealand 182 

Clinton mill, Amador Co., Cal 58 

Clunes district, Victoria 102 to 118 

Colorado milling practice, compared to California 2, 5, 69 

described 11 to 34 

growth of 8 

compared to concentration 7, 33 

comparative table of mills 15 

Columbus miU, Central City, So. Dak 78 

Comer mill, Thames District, New Zealand 169 

Comparative tables . . 15, 36, 58, 80, 105, 121, 135, 155, 169, 198, 209, 224 

Complex ores milled 168, 174, 197, 201 

Concentrates, treatment of chlorination 162, 203 

pans 165, 172, 177, 201 

cyanidation 202 

Concentrating machines, bumpers 18, 30, 90 

buddies 104, 109 

blankets ; see blanket tables. 

Halley percussion tables ..... 138, 141 

Frue vanners 38, 47, 49, 70, 128 

Triumphs 38, 49, 70 

Rittinger tables 38 

Woodbury 61, 70 

Concentration, imperfect 90, 92, 130 

versus stamps 7, 33 

Costs of milling 21, 45, 66, 71, 100, 112, 143, 172 

comparisons 221 

treatment of tailings 166, 178 

blankets . 188 



254 INDEX. 

Costs of hand-feeding , 32, 140 

screens 46, 52, 54, 188 

dies 46,206,211,221 

shoes 89,134,206,208,211 

mercury 46, 84 

water 45, 66, 172, 223 

Crushing affected by feeding 152 

coarse versus fine 2, 5, 60 

rapid S9 

fine a necessity 29, 60 

unequal 5 

rate of . . 15, 36, 58, 80, 105, 121,135, 155, 169, 173, 198, 224, 227 

Deadwood-Terra Company's mill, So. Dak 77, 84, 91, 98 

Dies, cast iron 16, 39, 65, 93, 157, 170, 208, 211 

chrome steel 66, 93, 208 

other steel 134 

wrought iron 108, 165, 211, 212 

use of false bottom 105, 186 

idea of the die 1 

cost of 46, 206, 211, 221 

Discharge, attempt at uniformity 81, 116 

depth of 16, 38, 46, 52, 135, 155, 169, 185, 226^^/ 

depth exceeds drop 67, 227 

intention of 4 

variation in 127, 130, 156, 173 

Discovery of gold, the Black Hills, So. Dak 75 

Bendigo, Victoria 133 

Ballarat, Victoria 119 

California 35 

Colorado 11 

Thames District, New Zealand 167 

New Zealand 184 

Victoria 102, 154 

Districts, Amador County, Cal 57 to 74 

Ballarat, Victoria . , 119 to 132 

Bendigo, Victoria . . . .' 133 to 153 

Black Hills, So. Dak 75 to 101 

Charters Towers, Queensland 198, 202 

Clunes, Victoria 102 to 118 

Gilpin County, Colo 11 to 34 

Grass Valley, Cal 35 to 56 

Harrietville, Victoria 154 to 166 

Nenthorn, New Zealand 193 

Reefton, New Zealand 197 

Thames, New Zealand 167 to 183 

Otago, New Zealand 184 to 195 

Dixon North Clunes mill, Victoria 105, 113 

Double-discharge mortars 106, 108, 116, 154, 162 



>, ;. INDEX. 255 

Drawings, amalgamating tables 48, 63 

amalgamating barreL 110 

mills 88, 73, 88, 94, 149, 288 

mortars 41, 68, 107, 150, 166 

screens 42, 156 

Drop, height of 16, 37, 60, 81, 108, 121, 135, 155, 169, 185, 226 

interval between 1, 29 

order of 15, 38, 89, 108, 120, 134, 139, 140 

less than depth of discharge 67, 227 

Elementary principles 1,8 

Elmore mill, Idaho 232 

Empire mill, Grass Valley, Cal 36, 46 

End-discharge mortars 156, 162 

Excelsior mill, Queensland 203 

Father De Smet Co.'s mill. Black Hills, So. Dak 76,87 

Feeding machines, necessary use . . 32, 150 

simple type of 163, 186 

variety of 82 

versuB hand work 31, 124, 148 

cost of 32, 148 

Feeding, too high 175 

Fine crushing a necessity 29, 60 

Fine crushing versus coarse 2, 5, 60 

Fortuna mill 134, 136 

Frue vanners 38, 47, 49, 70, 128 

Gilpin County district, Colo 11 to 34 

Gold, discovery of; see discovery. 

Gold, in ores, coarse 114, 137, 145, 194 

fine but free 43, 60, 98 

fine and not free 25, 177 

Gold production of Amador County, Cal 68 

Ballarat, Victoria 119 

Bendigo, Victoria 133 

Gilpin Co., Colo 12 

Grass Valley, Cal 36 

Thames, New Zealand 167 

Otago, New Zealand 184 

Golden Star miU, Black Hills, So. Dak 77, 84, 89 

Gover mill, Amador County, Cal 58, 71 

Grass Valley district, Cal 35 to 56 

Gregory Bobtail mill, Gilpin County, Colo 15, 21 

Grizzlies 37,148 

Guides . . . . 93, 165 

Halley percussion tables . 138, 141, 147 

Hand-feeding 31, 124, 148 

Harrietville district, Victoria 154 to 166 

Harrietville mill, Victoria 155, 160 

Hauraki ; see Thames. 



256 HTDBX. 

Heavy Btamps .... 6 

Hidden Treasure mill, Gilpin County, Colo 14, 21 

Highland mill. Black Hills, So. Dak 77, 89 

Hillsborough mill, Victoria 155 

Homestake Co.'s mills, Black Hills, So. Dak 77, 84, 89 

Idaho mill. Grass Valley, Cal 36, 51 

Interval between drops 1, 29 

Iron-framed batteries 144, 159 

Kennedy mill, Amador Co., Cal 58, 70, 71 

Keystone mill, Amador Co., Cal 58, 71 

Kuranui mill, New Zealand 169 

Lady Barkly mill, Victoria 135, 137 

Lime, use of 124, 125, 127 

Lining for mortar 95, 157 

Loss in tailings 143,146,163,177,183 

Mercury, addition of 44, 83, 158 

loss of . . . .25, 51, 61, 84, 106, 111, 131, 137, 142, 158, 169, 231 

flouring of 188, 216, 218 

sickening of . : . . . 174, 216, 219 

traps for 91, 108 

cost of 46, 84 

Mills, Alaska Treadwell 225 

Brittania United 121, 125 

Burdekin 203 

Caledonia 76 

Columbus 78 

Clinton 58 

Catherine 135, 137 

Cambria 169, 182 

Comer 169 

Deadwood-Terra 77,89,91,98 

Dixon North Clunes 105, 113 

Elmore 232 

Empire 36, 46 

Excelsior 203 

Father De Smet 76, 87 

Fortuna 134, 135,136 

Gover 58, 71 

Gregory Bobtail 15, 21 

Golden Star 77, 84, 86, 89 

Highland 77, 89 

Homestake 77, 84, 89 

Hidden Treasure 14, 21 

Hillsborough 156 

Harrietville 156, 160 

Idaho 36, 51 

Kennedy 58, 70, 71 

Keystone 58, 71 



INDEX. 257 

MiUs, Kuranui 169 

Lady Barkly 135, 137 

Moanataeri 169, 173 

New Chum Consolidated 135, 140 

New York 15, 21, 22 

North Star ' 36, 46 

New Normanby 121, 127 

North Cornish 121, 128 

Oriental 155, 163 

Phoenix 184 

Premier 185, 192 

Pearl 135, 138 

Port Phillip 102, 105, 199 

Prize 15, 21 

Plymouth 62, 72 

Reliance 185 

Randolph 15, 21 

Railway 155 

South Clunes . 105, 112 

Stephens 155 

Star of the East 121, 124 

South Spring Hill 58, 69, 71 

Wildman 57, 70, 71 

W. Y. O. D 36, 50, 56 

Moanataeri mill, New Zealand 169, 173 

Mortar, deep versus shallow 5 

deep and narrow 82, 94, 95 

deepening in California 55 

double discharge 106, 108, 116, 154, 162 

end discharge 156, 162 

wide versus narrow 138 

widening at Bendigo 141, 147 

too wide 170, 175 

poor design 191 

lining of 95, 157 

Muntz metal plates 176, 179 

Nenthorn district, Otago, New Zealand 193 

New Chum Consolidated mill, Victoria . 135, 140 

New Normanby mill, Victoria 121, 127 

New York mill, Gilpin Co., Colo 15,21,22 

North Cornish mill, Victoria 121, 123 

North Star mill, Grass Valley, Cal 36, 46 

Ore-bins 37,87 

Ores, clayey 159 

complex in andesite 168, 174 

complex sulphides 197, 201 

heavily pyritic 25, 202 

quartz in slate 66 



258 INDBX. 

Ores, quartz in slate and sandstone 145 

quartz schist 76, 98, 189 

simple quartz 114, 189 

character determines treatment 191, 232, 233 

variety treated 167, 236 

very rich 168 

Oriental miU, Victoria 166, 163 

Otago district. New Zealand 184 to 195 

Pans, Berdan ; 110, 123, 192 

Price 172 

Wheeler 121, 163, 165 

Watson & Denny 172 

Pearl mill, Bendigo, Victoria 136, 138 

Phoenix mill. New Zealand 184 

Plates (see also amalgamating tables), action of acid water ... 82 

dressing of 20, 182 

inside mortar 17, 40, 81 

of copper 17, 82, 123, 136, 192 

of silver-plated copper 40, 15S 

of Muntz metal 176, 179 

copper versus plated 158 

Plymouth Con. Co.'s mill, Amador Co., Cal 62, 72 

Pockets of rich ore, treatment 169 

Port Phillip mill, Victoria 102, 105, 199 

Premier mill, Otago, New Zealand 186, 192 

Prize mill, Gilpin Co., Colo 16, 21 

Pyritic ores treated 25, 202 

- Quartz ores 66, 114, 189 

Quartz schist ores 76, 98, 18^ 

Railway mill, Victoria 155 

Randolph mill, Gilpin Co., Colo 15, 21 

Rate of crushing .... 16, 36, 58, 80, 105, 121, 137, 166, 169, 173, 198, 224 

Reefton district, New Zealand 197 

Reliance mill, Otago, New Zealand 185 

Retorting 20, 122, 231 

Rittinger table 38 

Rock-breaker, advantage of 152, 175, 213 

Gates breaker 79 

erection at mine 79 

use in Victoria 118, 131 

not used 31, 60, 175 

Sampling 50,94,241 

Screens, aluminum bronze 83 

breakage of 60, 83 / '. 

action of acid water upon 23, 24, 176 

burr slot 16 

brass wire 51 

changed to suit ore 60 



Of^^vi -p S -l^x^tf^,,^ ^^ // ^^ 



6> ' 



IKDEX« 259 

Screens, copper plate 105, 112 

comparisons 228, 229 

cost of 46,62,54,188 

partitions 40, 42 

round-punched iron .... 129, 135, 140, 167, 170, 173, 185, 188 

steel cloth 63, 186, 188 

steel plate 136 

tin plate 48, 62 

wear of 135, 137, 140, 162, 167 

Shafting, arrangement of 89 

Shoes, cast iron 16, 56, 108, 167, 170, 208, 212 

chrome steel 39, 66, 207 

cost of 39, 122, 131, 206, 211 

ferro-alumina 93 

steel 93, 208 

idea of the shoe 1, 206 

wrought iron 166 

Slime tables . 66 

Sluice-plate, absurdity of 43, 71, 72, 100 

arrangement of 239 

South Clunes mill, Victoria 105, 112 

South Spring Hill mill, Amador Co., Cal 58,69,71 

Specimen ore 172 

Splash-board ' . . 165 

Stamps versus concentration 7, 33 

Stamps, weight of 137, 138, 225^ C 

Star of the East mill, Victoria 121, 124 

Steam stamps 97 

Stephens mill, Victoria . . . 156 

Tables for amalgamation ; see amalgamating tables. 
Tables ; see comparative tables. 

Tailings, contents of 143, 146, 163, 177, 183 

treatment of 65, 162, 166, 178, 234 

Tappets 14, 94, 121, 160 

Temperature, effect on amalgamation 126 

of battery water 92, 125 

Tests 27, 163, 183 241, 242 

Thames district. New Zealand 167 to 183 

Treatment of blanketings 65, 111, 141, 171, 187, 192 

concentrates 162, 165, 172, 177 

tailings 65, 162, 165, 178, 234 

Triumph concentrator 38, 49, 70 

Uniformity of discharge 81, 116 

Water, adjustment of 164, 194 

conservation of 86 

cost of 45, 66, 172, 223 

consumption of 22, 61, 61, 80, 122, 187, 232 

intention in its use 2 



260 INDBZ. 

Water, temperature 92, 125 

Weight of stamps 137, 138, 225 

Wells, compared to plates 116, 124, 217 

use of 136, 138, 164 

unsuitability to certain ores 171, 176 

Wildman mill, Amador Co., Cal 58, 70, 71 

Woodbury concentrator •...., 61, 70 

W. Y. O. D. mill , Grass Valley, Cal 36,50,56 



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The Mineral Industry, 



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IN THE 



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FROM THE EARLIEST TIMES. 







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" 1895, 


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" 1897, 


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ID 



Vol. I. From the Earliest Times to the Close of 1 892, 

Vol. II. SupplementinfiT Vol. 1 

Vol.111. " Vols.I&n " 

Vol. IV. " Vols. I, II & III •* 

Vol.V. " Vols. I to IV *• 

Vol. VI. •• Vols. I to V 

BY 

RICHARD P. ROTHWELL, 

Editor of the Engineering and Mining Journal, 
£x-Pr§$ident American Institute of Mining Engineers, Member American Society of CiwI 

Engineers, Fellow Royal Statistical Society, etc., etc 



These are the most thorough and exhaustive works published In any 
lanffuaffe on minlnff* metallurgy^ markets, and uses of the commercial 
minerals and metals. 

Every one interested in knowinij: the very latest and best methods In 
use for mininff* extracting, and refining the useful minerals and metals* 
and the amounts and values of each produced and consumed in every 
part of the world, can find the information in these annual volumes. 

To the engineer, the chemist, the metallurg^ist, the buyer, the seller of 
minerals and metals, the investor in mineral property, and to the legis* 
lator, who should know the resources and conditions of production in 
every country to lefirislate wisely for his own, they are absolutely indis« 
pensable. 

Each volume is complete in itself. The information contained In one Is 
supplemented but not repeated in the other. 



THE SCIENTIFIC PUBLISHING CO., Publishers, 

NEW YORK: 253 Broadway. LONDON: 20 Bucklersbury. 



A WELL SELECTED LIST 



OF 



TECHNICAL BOOKS 



PUBLISHED BY 



THE SCIENTIFIC PUBLISHING COMPANY, 



NEW YORK: 253 Broadway. 



LONDON: 20 Bucklersbury 



Every book in this list is the dominant authority in 
line, and we should be pleased to forward a descriptive 
application: 

Engineering and Mining Jonrnsl (weekly) per year, ... 

The Mineral Industry : an Encyclopedia of Mining and Metallurgy. 

Vol. I. Statistics to the end of 1892, 
Vol. II. Statistics to the end of 1898, 
Vol. III. Statistics to the end of 1894, 
Vol. IV. Statistics to the end of 1895, 
Vol. V. Statistics to the end of 1896, 
Vol. VI. Statistics to the end of 1897 

Universal Bimetallism. By Richard P. Roth well, .... 

Stamp Milling of Gold Ores. By T. A. Rickard, 

Modern Copper Smelting. By E. T>' Peters, Jr., • 

Matte Smelting. By Herbert Lang, ..... 

Notes on Lead, Copper Smelting and Copper ConTerting. By H. W. Hlxon, 

The Metallurgy of Lead. By H. O. Hofman, .... 

The Metallurgy of Steel. By Henry M. Howe, ... 

Manufacture and Properties of Structural Steel. By H. H. Campbell, 

Lead and Zinc Mines of Missouri and Kansas. By J. R. HoUbaugh, 

The Phosphates of America. By Francis Wyatt, 

Ore Deposits of the United States. By J. F. Kemp, 

Manual of <{nalitatiTe Blowpipe Analysis. By F. M. Endlich, 

Outline of Qnalitative Chemical Analysis. By J. M. Miller, 

A New Basis for Chemistry. By Thomas Sterry Hunt, 

Chemical and Geological Essays. By Thomas Sterry Hunt, 

Systematic Mineralogy. By Thomas Sterry li ^t. 

The Canadian Ice Age. By Sir J. William Dawson, ... 

Gems and Precious Stones of North America. By G. F. Kunz, 

Mining Accidents and their Prevention. By Sir F. A. Abel, 

Columbian Parliamentary Compend or Rules of Debate. By H. W. Hoot, 

The Mining Laws of the Republic of Colombia. Translated into English, 

SOLE AMERICAN AGENTS FOR 

Report Book for Mining Engineers. By A. G. Charleton, 

Mining and General Telegraphic Code. By Bedford McNeill, 

Florida, South Carolina and Canadian Phosphates. By C. C. Hoyer Millar, 



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